Not only do Democratic defections nearly double in a McCain v. Obama race, but Obama loses a fifth of white Democrats to McCain, and he runs seventeen points behind Clinton among
Most remarkable of all is that Obama is weaker among Democrats in all age groups than Clinton. He is four points weaker, and McCain five points stronger, among Democratic voters aged 18-49 than in a Clinton v. McCain race. The losses are even greater among Democratic voters 50-64 and 65+. Democratic defections increase across income groups as well ... And those “Obamacans” we keep hearing about? They do exist, making up 8% of Republicans (three points higher than Clinton), but they are hardly the stuff of historic realignment and they are outnumbered almost two-to-one by “McCainocrats.”
Despite all this, Obama still enjoys a seven point lead over McCain, 50-43 percent, which as Daniel notes is almost entirely due to his support among independents and young voters. (Though to be fair, one reason there may be fewer "Obamacans" than you'd expect is the recent and sudden collapse of the GOP brand; a lot of people who might have called themselves "Republicans for Obama" now presumably call themselves independents.)
This landscape, if I may return to one of my hobby-horses, is exactly why Obama is such a high-risk, high-reward candidate for the Dems. He has the potential to do for the Democratic Party what Reagan did for the GOP in '80 - to win a lopsided victory in which a slew of previously-wavering independents and politically-unformed twentysomethings end up branding themselves as Democrats for a generation. That's the good news; the bad news is that if he doesn't win a lopsided victory among independents and young voters - if the bloom comes off the rose or the glass jaw starts to crack - he has the potential to hemorrhage votes in key constituencies: among downscale voters; among seniors (where I suspect the "wouldn't vote for an African-American" constituency is concentrated); among hawkish and extremely pro-Israel Dems; and even among white women. Which is to say, he could win in a walk, or lose thanks to heavy defections from groups that would have trended Democratic had Hillary been the nominee.