Paul Krugman blames Obama's supporters for the poisonous state of the Democratic race:

[M]ost of the venom I see is coming from supporters of Mr. Obama, who want their hero or nobody . . . progressives should realize that Nixonland is not the country we want to be. Racism, misogyny and character assassination are all ways of distracting voters from the issues, and people who care about the issues have a shared interest in making the politics of hatred unacceptable.

H/T. But Frank Rich puts the blame on race-baiting by the Clintons (I guess Rich finally woke up to the idea that there could possibly be reasons to oppose the Clintons that don't derive from one's sexual hang-ups or whatever his theory was in the 90s):

The campaign's other most potent form of currency remains its thick deck of race cards. . . . This decision was a cold, political cost-benefit calculus. . . . [O]nce black voters met Mr. Obama and started to gravitate toward him, Bill Clinton and the campaign's other surrogates stopped caring about what African-Americans thought. In an effort to scare off white voters, Mr. Obama was ghettoized as a cocaine user (by the chief Clinton strategist, Mark Penn, among others), "the black candidate" (as Clinton strategists told the Associated Press) and Jesse Jackson redux (by Mr. Clinton himself). . . [T]he wholesale substitution of Hispanics for blacks on the Hallmark show is tainted by a creepy racial back story. Last month a Hispanic pollster employed by the Clinton campaign pitted the two groups against each other by telling The New Yorker that Hispanic voters have "not shown a lot of willingness or affinity to support black candidates." Mrs. Clinton then seconded the motion by telling Tim Russert in a debate that her pollster was "making a historical statement."

It wasn't an accurate statement, historical or otherwise. It was a lie, and a bigoted lie at that . . . The real point of the Clinton campaign's decision to sow misinformation and racial division, Mr. Rodriguez concluded, was to "undermine one of Obama's central selling points, that he can build bridges and unite Americans of all types." . . . Mrs. Clinton did pile up her expected large margin among Latino voters in California. But her tight grip on that electorate is loosening. . . .Meanwhile, the Clinton campaign's attempt to drive white voters away from Mr. Obama by playing the race card has backfired. . . . The question now is how much more racial friction the Clinton campaign will gin up if its Hispanic support starts to erode in Texas . . . [D]oes anyone seriously believe that Howard Dean can deter a Clinton combine so ruthless that it risked shredding three decades of mutual affection with black America to win a primary?

Read the whole, wonderful thing. This Democratic race really can't go on long enough, can it?