A new poll shows that 20 percent of Barack Obama’s supporters would vote for John McCain if Hillary Clinton wins the Democratic nomination. Similarly, 19 percent of Hillary’s supporters would vote for McCain should Obama be running against him. And people thought the GOP was split!

This fracturing in the Democratic Party is yet another result of identity politics. Once people have chosen a candidate solely because of gender or race, there’s no ideological commonality to reconcile different voter blocs within the party. If a Hillary voter’s favorite Hillary policy is that she’s a woman, what need is there of Obama’s healthcare plan? The same dynamic holds on both sides. If an Obama supporter is solely concerned with his candidate’s racial composition, what sway does Hillary’s troop withdrawal hold?

The nasty tenor of the Democratic race is another factor, and here the blame lies squarely with the Clinton camp. From the start, Bill Clinton has missed no opportunity to demean Obama, either by diminishing his credentials or reducing him to a black phenomenon (by, say, comparing his South Carolina victory to Jesse Jackson’s). Hillary picked up where Bill left off. She made too much of Obama’s relationship with Tony Rezko, and she shamefully played up any reference to race or cultural exoticism.

This is ugly stuff. At least the Republican crack-up-that-wasn’t was ideological. Rush Limbaugh and company pointed to a handful of policy decisions they felt rendered John McCain unfit. This meant there was room to patch things up. When dealing in degrees of political conservatism, one side can budge, the other can give, a few pledges can be made, and so on. Hillary Clinton can’t turn black and Barack Obama can’t become a woman. Even the most fervent “change”-peddler wouldn’t promise that much. (At least until Pennsylvania.)