Among likely Pennsylvania Democratic primary voters, Obama trails by 13 and has slightly higher unfav ratings (27 to Clinton’s 22).  To address Noam Scheiber’s point about a different Pennsylvania poll, it seems that there is a sizeable number (30%) of white Democrats who are unlikely or very unlikely to vote for Obama against McCain in the general.  According to Rasmussen, even 8% of black Democrats are unlikely or very unlikely to vote for Obama against McCain.  17% of white Democrats and 23% of black Democrats are unlikely or very unlikely to vote for Clinton against McCain.  Obama’s unfavs among white Democrats are 33%, which is comparable to Clinton’s unfavs among black Democrats (30%).  Overall, 78% of respondents say they are somewhat or very likely to vote for Clinton against McCain, and just 71% say the same about Obama.  18% are unlikely/very unlikely to vote for Clinton, while 26% say the same about Obama.   

As the general election polling from Pennsylvania makes clear, Obama does as well as he does in the match-up with McCain because he draws enough Republican and independent support to offset the Democratic defections that he would likely suffer.  Clinton draws fewer non-Democrats, but retains enough Democrats to do slightly better against McCain than Obama according to current preferences.