Here's what the Democrats argue:

"The fact is, there are currently many more Democrats registered in battleground states now than there were before the 2008 primary campaign began, which means there are fewer eligible voters left to register because of the gains we made in 2008," campaign spokesman Adam Fetcher said in an e-mail.

"We have largely preserved the huge gains we built in 2007 and 2008 and increased our advantage in some areas..."

Now, as a matter of logic and basic statistics, it's true that when you pick a lot of the low-hanging fruit in one year, the ceiling on your rate of growth the next time around will be a lot lower. In some groups, like African-American adults (ie, born before 1987), this is undoubtedly a partial explanation.

But it also misses a basic dynamic regarding the Democrats' coalition. The Democrats in general, and Obama in particular, rely on winning college-age voters by large margins. According to the NY Times exit polls, in the past 4 elections, Democrats carried the 18-29 year old vote 53-34 (Clinton vs Dole), 48-46 (Gore vs Bush), 54-45 (Kerry vs Bush) and 66-32 (Obama vs McCain). In the same four elections, Democrats won first-time voters 54-34, 52-43, 53-45 and 69-30. CNN's exit polls had Obama winning voters age 18-24 (10% of the electorate) by 66-32 and 25-29 (8%) by 66-31, while McCain won a slight majority among the 64% of the voters age 40 and over. (The gap was especially pronounced among white voters, who went for Obama 54-44 in the under-30 age cohort but which McCain won by some 17 points among white voters over 30). Both sets of exit polls pegged new voters at 11% of the 2008 electorate.

Here's the thing about voters age 18-21: there is 100% turnover among the group in each presidential election. Voters age 18-21 in 2008 were born between 1987 and 1990. Many of them were college students who probably registered at either their college address or their parents' home. Most are now out of college, and may have moved away from home; in a fairly sizeable number of cases, those voters will need to update their registrations or re-register, or they are lost to the Democrats. Even if they do re-register, Democrats will need to be updating their databases to do GOTV efforts. Chasing young voters costs money that needs to be re-spent every cycle. The core Republican vote of adult homeowners with jobs and families does not present this problem.

By contrast, voters who are age 18-21 this year were born between 1991 and 1994, and - to put a fine point on it - not one of them was registered in 2008. If Democrats are not showing a large volume of voter registration, they are failing to register young voters who will be direly needed to win this election. This is aside from the fact that there has also been an unusual amount of voter dislocation since 2008 due to the economy - homes foreclosed, people moving to look for work. That can affect both parties, but it suggests some caution in Democrats relying on just maintaining their 2008 voter rolls.

This may prove particularly important because turnout will be so critical to this year's election, perhaps even moreso than in 2004. If you read the polls, you have to be struck by the wide gaps between the polls of registered voters or all adults, those that attempt to model the election to resemble the 2008 turnout, or those that assume the electorate will look something like a cross between the 2008 and 2010 electorates. I don't doubt that most of the pollsters are honestly reporting the results they are getting, but they diverge when they apply their as-much-art-as-science judgments to who is actually going to vote.

In the end, nobody knows for sure just yet. And even the voter registration picture may look a lot brighter for Democrats when college students return to campus in the fall. But the registration picture painted by the Globe is inarguably bad news for them as it stands right now.

Identity Politics:

Gender: In theory, given that vice presidents have a fairly good chance of becoming president but little proven ability to affect the election (Bush-Quayle is the most well-known example of a ticket that won despite general consensus that the VP was a political liability), we should want the choice of vice presidents to be made solely on the basis of merit. But running mates are the one and only prominent hire made by the candidate during the heat of a contested election, and so almost invariably they are chosen with a lot of emphasis on political considerations; even Dick Cheney, the most obvious "screw politics, I'm picking the best guy for the job" choice, was well-matched to George W. Bush's major perceived weakness at the time (lack of foreign policy gravitas).

So, we can't discount politics, and that includes identity politics, which like it or not has always been a part of appeals to voters wherever there has been elections. That's not to say a responsible campaign should let identity politics trump other considerations, simply that there's no sense in pretending that it will not, in the real world, factor into decisionmaking.

The biggest potential target is women, who constitute a majority of the electorate (only white voters and Christians are a larger faction, and both are even less likely than women to see themselves as a cohesive voting bloc). All things being equal, Mitt Romney would clearly love to select a female running mate, as he did the one time he had the chance to do so before (his Lieutenant Governor in Massachusetts, Kerry Healey). But the problem with the available choices is that they are an even more extreme illustration of the transitional nature of the GOP's talent pool in general: the experienced women, like Condi Rice and Kay Bailey Hutchison, are pro-choice and/or otherwise politically unacceptable to the party, whereas the more conservative women elected in 2010, like Kelly Ayotte, Susanna Martinez and Nikki Haley, are mostly not quite ready for prime time yet. In Martinez' case, she's been all but screaming from the rooftops that she doesn't want the job, and as a result almost certainly has not been vetted by the Romney campaign. In between, the closest to women with the requisite level of both experience and ideological positioning would be Congresswomen Cathy McMorris Rodgers of Washington and Marsha Blackburn of Tennessee, and I've seen no indication that either has drawn much support or attention in this process.

Ayotte is the closest to a credible candidate - she ran the state Attorney General's office in New Hampshire, she earned street cred with pro-lifers by fighting an abortion case (on parental notification) all the way to the Supreme Court and winning, she's campaigned early and often with Romney dating back to the primaries and has clearly got a good rapport with him on the trail, and she looks and sounds like the person you'd cast as a busy suburban professional woman in a minivan commercial. In 2010, she carried independents by 26 points in a state that had voted for both Obama and Kerry and was in the process of re-electing its Democratic governor. But while New Hampshire is one of the states Romney still hopes to swing, Ayotte is still a comparatively unknown figure from a tiny New England state in Romney's back yard. And the shadow of the Palin controversies still loom: while Ayotte is stylistically nothing like Palin's red-moose-meat persona, the perception of Palin as an underqualified running mate would require Ayotte to meet a higher standard than a man to convince the media that she's ready for the job.

Race/ethnicity: The talent pool of Hispanic or African-American Republicans has the same basic problems - burned-out veterans who are non-starters (Rice, Colin Powell, Mel Martinez, etc.) and young up-and-comers who would have to overcome short resumes in major public office (Marco Rubio, Susanna Martinez again, Allen West). Rubio, of course, is a great package of domestic and foreign policy conservatism, eloquence, a great life story and popularity in a huge swing state, and he's a more seasoned figure than Obama was in 2008 - in the Florida Legislature he was Speaker of the House, not an obscure backbencher - but he's happy in the Senate and seems interested in building a career there before reaching for a national perch (about the only argument for running now is his hairline). Romney's been wisely hesitant to rule out Rubio completely; he'd be the most dramatic choice and the one you would reach for - as McCain reached for Palin - if you thought you were losing but not yet defeated and needed to go big and try to run the table the rest of the way. But while the polls have not been encouraging the past few weeks, there's no sign that Romney is thinking that way.

Also, it's not at all clear that non-Cuban Hispanic voters in key swing states are open to Romney or would respond to the Cuban-American Rubio; everything we have seen from Romney indicates that his political calculus is focused on counter-programming Obama's racial divide-and-conquer strategy by picking up white swing voters in the mostly Midwestern states that are still overwhelmingly white. There are six potential swing states where the electorate, based on Census data and past elections, is likely to be more than 80% non-Hispanic whites, and not coincidentally, those were the six states on Romney's summer bus tour: New Hampshire, Pennsylvania, Ohio, Michigan, Iowa, and Wisconsin. Colorado's population is nearly 90% white according to the Census, but that includes Hispanic voters; Virginia's is below 80%:

It's distasteful to carve up the electorate on those lines, but certainly the demographics of those states are an argument against picking a Latino running mate solely for the sake of picking a Latino running mate.

South Asian voters are a much tinier voting bloc, but they happen to have one of the remaining talked-about candidates in Bobby Jindal. If Romney is looking to appeal to voters who are generally uncomfortable voting for a ticket of two white males, Jindal - who's overqualified for the job for multiple other reasons - could solve that problem for him.

Religion: Then there's that other identity-politics wild card: religion. On the one hand, Catholics are a potentially important swing vote, albeit a group that in many cases will not vote as Catholics. (Also, according to the NY Times exit polls, Obama lost white Catholics 52-47 four years ago and no Democratic nominee has won more than 48% of white Catholics since Jimmy Carter in 1976, so the "swing" part may be overrated). On the other hand, there has never in American history been a major-party presidential ticket without a Protestant (the Democrats have had a Catholic on the ticket seven times - Al Smith, JFK, Muskie, Sergeant Shriver, Geraldine Ferraro, John Kerry and Joe Biden - and a Jewish candidate once - Lieberman - and the GOP has run a Catholic VP candidate once, William Miller in 1964), and a ticket combining Romney with a Catholic like Rubio, Paul Ryan, Jindal, Chris Christie or Bob McDonnell would set sail for that uncharted territory. That's especially true at a time when the leadership of the House and Senate are very thin on Protestants and there are no Protestants on the Supreme Court. For Romney, perhaps still nervous about the unpopularity of his own Mormon faith, that may counsel in favor of the Methodist Rob Portman or the evangelical Christian Tim Pawlenty.

Class: Like it or not, Mitt Romney is heavily identified with being a rich son of a rich CEO who grew up to be a rich CEO himself; Americans love rags-to-riches stories, but are not so much enamored of riches-to-more-riches stories in their leaders (not that this has prevented them from repeatedly electing Roosevelts, Kennedys, Bushes, etc.) Still, with Obama hammering him over his money, his taxes, and his role as CEO, voters who have never had anything resembling Romney's money are clearly concerned that Romney may just not understand what it's like to not be rich. That gives an advantage to candidates who have had something of a rougher ride in life, whether they be the sons of immigrants (Rubio, Jindal), or people who went through hard times (Ryan, whose father died suddenly when he was 16; Pawlenty, the son of a truck driver, lost his mother to cancer as a teen).

Geography: Geography is a little different from identity politics, but perhaps not so much; it's even more of a traditional consideration in Vice Presidential politics, and in point of fact, many people are far more apt to react negatively to a politician who is seen as too Southern or too Northeastern than to distinctions of race, gender or religion. As noted above, with a handful of exceptions (New Hampshire, Colorado, Nevada, Florida, Virginia), by far the dominant region up for grabs in this election is the Midwest, which is undoubtedly one of the several reasons why most reports seem to indicate that the frontrunners for the VP slot come down to three Midwesterners - Ryan from Wisconsin, Portman from Ohio, and Pawlenty from Minnesota, a state that is probably out of Romney's reach but similar in other ways to the neighboring battlegrounds. Portman probably has the edge on geography alone - Ohio is a perennial swing state that no Republican has ever won without, and while Portman's name recognition is fairly low even in his home state, he does have a solid organization in key parts of the state that helped Romney pull out a hard-fought primary victory over Rick Santorum. Geography is also almost the sole reason why McDonnell has entered VP discussions, although he seems a long shot now for a variety of reasons. The candidate who is hurt most by the geography issue is Jindal, the lone Deep South candidate under consideration, albeit a very un-stereotypical Southerner in nearly every way but his accent and his politics.

Age: This is an under-discussed angle to the VP selection, but one that figured heavily in the choices of FDR in 1920, Dan Quayle in 1988, Al Gore in 1992, and Palin in 2008: the desire to either balance a ticket or provide (in the Clinton-Gore case) an overall reinforcing image of youthful vitality. Romney, like Clinton and George W. Bush, is a Baby Boomer, born in the 1940s; three of the candidates under discussion (Ryan, Jindal and Rubio) were born in the 1970s and came of age in the Reagan years. That could raise concerns in some voters about their readiness for the job, as it did with both Quayle and FDR, but it also sends younger voters a message that the GOP is not just your grandpa's political party.

The Candidate Factors:

Ideology: It's no secret that conservatives in general and social and fiscal conservatives in particular still mistrust Romney for his past positions on abortion, health care and a litany of other issues. Much of that has been set aside as the party unifies behind removing Obama, but it's never far from the surface, and as a result, Romney will face some disappointment if he picks a candidate who is viewed as generally moderate (such as Pawlenty, Ayotte or Christie), and open revolt if he picks a liberal and/or pro-choice Republican.

Experience: Romney's running as a Mr. Fix-It executive who values business experience, but he himself has never served in DC. There's some tension here: few of the people he's looking at have spent a lot of time in the private sector, but the experiences they offer are different. Pawlenty's been a two-term governor and state legislative majority leader, so he's clearly ready to be a chief executive but doesn't offset Romney's lack of foreign policy credentials or DC experience. Ryan has essentially no executive experience beyond being Budget Committee chairman, but has been in DC since the 90s and is an acknowledged domestic policy expert. Portman and Jindal have much more varied resumes, ideally suited to a presidential candidate (like Pawlenty, both have worked in both executive and legislative jobs, although Portman hasn't been a chief executive; Portman has more foreign policy experience, Jindal has the advantage of having served both in and out of DC).

Personality: Here's where the rubber starts to hit the road in a serious way. On the one hand, you have Portman and Pawlenty and longer shots like McDonnell and John Thune who are all sort of dry and considered boring. On the other hand, there's the concern - which should not be a problem for a confident candidate, but could figure nonetheless - that Romney could be outshone by a more colorful choice like Christie or Rubio. I think this is actually part of the constellation of reasons Christie won't be picked: his chief asset is that he's a leader and a take-charge personality who dominates every room he enters, and that's just not the kind of guy you pick for a second banana job.

Pawlenty presents the problem most dramatically at the other end. He's the candidate I originally supported in the presidential race in large part because he's the ideal check-box guy who has no real weaknesses. But his campaign suffered from the fact that he just does not project a forceful, authoritative personality, epitomized when he refused to attack Romney's health care plan to his face. If you think Romney is sailing ahead on course to win, he's the lowest-risk pick, and there's still something to the idea that Romney needs to keep his sails trimmed and let Obama beat himself. But even with all the methodological issues with the past few weeks of polling, I think it seems pretty clear by now that Romney still needs to sell the voters on himself. And Pawlenty just won't help him do that.

Why Not Jindal?

If I was choosing a 2012 GOP presidential nominee from scratch right now, my top choices would be Jindal - who would make the best potential president - and Christie, who is the man best suited to the times and would bring a powerful leadership style to the campaign and the job. I certainly won't be disappointed if Jindal is the surprise pick. But we're not picking a number one, we're picking a number two who might become a number one. And while this election is hugely important, it's still one we could lose, and we'd best keep some powder dry for 2016 just in case. Both of those candidates will be prime contenders then if we get there.

Jindal is a shake-it-up pick, and undoubtedly ready for the big job, but is he ready for the national campaign trail? He's a brilliant guy, but while governors make the best presidential candidates, they sometimes - as we saw with both Palin and Rick Perry - stumble if they get thrown straight onto the national trail and suddenly have to face a whole new set of reporters and a whole new set of issues that are different from their state-level political environments. (That's less of an issue for Pawlenty, who has been on the national campaign trail almost as long as Romney by now). Plus, a VP candidate can be at the mercy of the presidential candidate's staff, impairing his or her ability to clean up negative stories. There's a lot of risk there - and as long as there are other good candidates who might marginally help Romney more in a tight race, the party's interests are probably better served by letting Jindal come to the national stage on his own timetable. Paul Ryan, by contrast, is a campaign-trail ready veteran of the DC talk show circuit, is probably stymied from advancing much further within the House, and might have more trouble running an independent presidential campaign without much of a record as an executive. He has more to gain and less to lose than Jindal by going national now.

(A related consideration here that affects Portman, Christie, Rubio and Ayotte in particular: I'd like to see the GOP avoid doing what Obama did and putting at risk a whole host of Senate seats and Governorships with his appointments. Pawlenty is out of office and Jindal and McDonnell are term-limited, so we would lose a lot less if one of them moved out of his current role).

Why Ryan?

I haven't covered all the possible considerations here, but at the end of the day, the choice seems likely to come down to Ryan, Portman and Pawlenty. None are terrible choices, and it may well be that Portman is the Cheney-esque pick here. But were I advising Romney, I'd go with Ryan.

Romney can win this election, but there is little sign that he's winning it right now, and as the Wall Street Journal points out, Ryan is the guy best positioned to step into the leadership deficit created by Romney's approach to this campaign:

The case for Mr. Ryan is that he best exemplifies the nature and stakes of this election. More than any other politician, the House Budget Chairman has defined those stakes well as a generational choice about the role of government and whether America will once again become a growth economy or sink into interest-group dominated decline.

Against the advice of every Beltway bedwetter, he has put entitlement reform at the center of the public agenda - before it becomes a crisis that requires savage cuts. And he has done so as part of a larger vision that stresses tax reform for faster growth, spending restraint to prevent a Greek-like budget fate, and a Jack Kemp-like belief in opportunity for all. He represents the GOP's new generation of reformers that includes such Governors as Louisiana's Bobby Jindal and New Jersey's Chris Christie.

As important, Mr. Ryan can make his case in a reasonable and unthreatening way. He doesn't get mad, or at least he doesn't show it. Like Reagan, he has a basic cheerfulness and Midwestern equanimity.

As for Medicare, the Democrats would make Mr. Ryan's budget a target, but then they are already doing it anyway. Mr. Romney has already endorsed a modified version of Mr. Ryan's premium-support Medicare reform, and who better to defend it than the author himself?

There's a tension, of course, in Romney choosing a guy known for policy specifics when Romney himself - despite his love for data, facts and figures - has chosen as one of the central pillars of his campaign strategy to keep his message general and let the focus stay on Obama. (Romney does have plenty of substantive policy proposals, but many are not yet terribly detailed, and most voters would be hard-pressed to identify anything in particular he stands for beyond resisting tax hikes on business). Picking Pawlenty or Portman, despite their other virtues, would entail doubling down on this approach, and sending conservatives a "trust me" message that has already generated a lot of support but not much enthusiasm. But the time comes when you need to get at least a little bit bold in order to get your own side excited and get the marginal voter in the middle to pay some attention. Of the likely candidates, Ryan is the best guy for the job.

Romney-Ryan 2012.