Looking at the RealClearPolitics 3-way polling averages, 11 out of 12 have Bush with between 47 and 51% of the vote, and 8 of 12 have Kerry with between 47 and 49% of the vote. The latest Rasmussen tracking polls are consistent as well, showing Bush leading 47.9-47.1, 48.1-47.1, and 48.8-47.4 over the past three days (the most recent listed last). Which means, essentially, that we have both an incumbent and a challenger who have a fairly solid base of support entering the last two days of the campaign. I think most of us will agree that it is highly likely that Bush will poll at least 47% on Election Day, and equally highly likely that Kerry will poll at least 46% and probably at least 47% - thus, at least a decently close election remains likely, although we could still have a decisive popular-vote majority and/or an Electoral College landslide.

Recognizing the limits of historical analogies, what can we determine from this? I decided to take a look at the final election results for elections dating back to 1824, when they started keeping records of the popular vote. There have been 25 elections in that period in which an incumbent has stood for re-election; 16 have been re-elected, 9 have been voted out of office.

Strong Incumbents, Weak Incumbents
Obviously, a strong incumbent - if we define a "strong" candidate as one who finishes with at least 47% of the vote - is likely to be re-elected. How likely? All 16 who were re-elected had at least 47%, while only two incumbents who polled at least 47% were voted out, those being Ford in 1976 with 48% of the vote and Grover Cleveland in 1888, who won the popular vote with 48.6% and was voted down (if you want to quibble with my line-drawing - and I had to draw it somewhere - the one incumbent in the 46% range, Martin van Buren at 46.8% in 1840, went down to defeat). The only three presidents to be re-elected with below 50% were Harry Truman in 1948 (49.4%), Woodrow Wilson in 1916 (49.2%) and Bill Clinton in 1996 (49.2%).

What's interesting - and, in fact, what shows the limitations of historical analogies - is how few incumbents have lost races without a complete collapse in their support. Besides Ford, the other four incumbents to lose since 1900 got completely abandoned at the polls: Carter in 1980 got 41%, Hoover in 1928 got 39.6%, Bush Sr. in 1992 got 37.4%, and Taft in 1912 got 23.2% and finished third. Besides Cleveland and van Buren, the other two 19th century incumbents to lose also showed weakly, in both cases against candidates who beat them in the popular vote four years earlier: Benjamin Harrison drew just 43% in his 1892 rematch with Cleveland, and John Quincy Adams drew just 43.6% in his 1828 rematch with Andrew Jackson.

The average margin of victory for successful incumbents? 54.9 to 41.1 overall and 54.9 to 40.9 since 1900. The average margin of victory for successful challengers? 49.5 to 41.2 overall, and 48.5 to 37.8 since 1900.

Strong Challengers, Weak Challengers
The flip side is when, as this year, we have a strong challenger: six candidates have drawn at least 47% of the popular vote against an incumbent president, and all of them have won. Of those, only one drew less than 50% of the vote: Benjamin Harrison in 1888 with 47.8%.

Strong Incumbents, Strong Challengers
You can see where this is heading. In the five presidential elections in which an incumbent and a challenger were separated by 5 points or less, the incumbent won two (Truman in 1948 and Wilson in 1916); the challenger won three, Carter defeating Ford in 1976 and the two Cleveland-Harrison matches in 1888 and 1892; or, that's a two-to-one advantage since 1900. Not much you can learn there either way. For what it's worth, the average outcome was 47.6% for the incumbent and 47% for the challenger, or 48.9% for the incumbent and 47% for the challenger since 1900.

If you look at matchups of a strong challenger against a strong incumbent, there's only two historical precedents, both of them bad for the incumbent: 1888 and 1976.

Conclusion
Well, it should be pretty clear from all this that the history isn't all that enlightening; there's really only five campaigns out of 25, and maybe really only two, that give us any examples to study. But I do think the history is a useful caution about reading too much into the study of, for example, how late-deciding voters make their minds up. The fact is, the 1976 campaign is the only one in the last 50 years to look anything like this one, and the polling data from 1976 don't exactly support the notion that voters facing a choice between a strong incumbent and a strong challenger will swarm to the challenger at the end, as Ford's strength came from a late surge after never pulling better than 45% until the final poll of the election, when he pulled briefly ahead at 49-48 with a momentary surge that quickly subsided (link via this analysis). And Ford, you may recall, was a bit of a unique incumbent: he had never been on a national ticket before, and was running on a record of the first election after Watergate and the fall of South Vietnam.

In other words: tomorrow, history leaves us on our own. It's our job to make it.