Obama Goes Negative

I don't get why he's doing this:

Obama campaign aides scheduled the call to talk about Hillary Clinton's fantastical story about her breakneck race to shelter under sniper fire during a visit to Bosnia. You might think this would be the last story the Obama campaign would be pushing, because in Wednesday's debate the Senator mistakenly suggested his campaign had only discussed the issue because reporters had brought it up, not because they were trying to take advantage of Clinton's extended work of fiction. To push the story now would make Obama look even more insincere about that claim.