Take a minute.
That's right: He hasn't lost any support whatsoever. Clinton has lost it. The "bitter" story broke on April 11, and was turned into an ad by the Clinton campaign on April 13. Since then Obama has moved from a 7-8 point deficit to a 5-6 point deficit in the polls.
That's a margin-of-error change, of course, and the trend lines show that Obama and Clinton have both been basically flat for a few days. But flatness is surely bad for Clinton. There's a track record for voter-insulting gaffes wrecking a canddiate at the 11th hour. Gary Hart blew his last chance at the 1984 nomination when he made a joke about New Jersey and toxic waste right before the state's primary. Mike Dukakis was on track to win South Dakota until Dick Gephardt made fun of him for telling farmers to raise Belgian endives. And of course, Obama suffered a last-minute collapse in Ohio when Austan Goolsbee told Canadian officials that Obama's anti-trade rhetoric was campaign trail boilerplate.
So if this didn't hurt him... well, why? My theory is that Obama quickly won the spin war and convinced voters he was a well-meaning truth-teller. The "bitter" comment, instead of the much more insulting "cling" comment, became the story. The whole quote was packaged as St. Barack finally, finally telling PA Democrats (most of whom will be voting in Philadelphia and the more suburban southeast) the truth about Republican tactics. Meanwhile, Hillary Clinton raced into the spotlight to accuse him of "elitism" and to drink shots with the hoi polloi. One of the candidates looked honest, one looked opportunistic. It was the flip-side of the Goolsbee gaffe.
It's much less clear how the debate is playing out, even though Obama quickly won that spin war—instead of bragging about a victory, Clinton is defensively arguing that, no, the debate wasn't completely unfair. Some polls show Clinton slightly up, some show her slightly down.