Paul Krugman is worried that divisions within the Democratic Party will cost it the presidency in an otherwise favorable year:

Here’s the point: the nightmare Mr. Obama and his supporters should fear is that in an election year in which everything favors the Democrats, he will nonetheless manage to lose. He needs to do everything he can to make sure that doesn’t happen.

I’ve also suggested that the division caused by the primary campaign may hurt the Democrats. On the other hand, as John Sides points out in a Los Angeles Times op-ed, “the reality is that presidential campaigns tend to unify each party behind its nominee.” Consider the 2000 election:

Early on, Democrats and Republicans appeared less than fully enthusiastic about their candidates. For instance, in June 2000, only 71% of likely Democratic voters said they would vote for Gore in the general election, according to the National Annenberg Election Study, a survey conducted by the Annenberg Public Policy Center at the University of Pennsylvania. The rest said they would either vote for Bush or another candidate, or were undecided. Among Republicans, only 82% said they planned to vote for Bush. But voters in both parties overcame or set aside their early doubts as the campaign unfolded. According to the Voter News Service’s election-day exit poll, 86% of Democrats voted for Gore and 91% of Republicans for Bush. Most partisans rejoined the fold.

Sides concludes by suggesting that even those Hillary supporters with race-related qualms about Obama (see the latest Newsweek poll for more) will mostly end up voting for him due to increasing polarization between the parties and the high salience of partisanship. That may be true, but as I wrote before, I still think Obama is likely to underperform relative to what political science models predict. If that happens, expect Hillary to get the blame even if it’s a largely inevitable result of racial factors.