The latest Pennsylvania poll, based on interviews conducted over the weekend, shows Hillary Clinton with a shocking 20-point lead over Barack Obama, 57%-37%, with only 4% undecided. Until last weekend, the trend had been running in Obama's favor, so it's hard to attribute the turnaround to anything other than Obama's "bitter" gaffe. Or maybe to Obama's advertising blitz, as 23% of likely Democratic primary voters said that excessive exposure to Obama's ads is making them more likely to vote for Clinton.

It may be lucky for Obama that he has another week to go before the primary. So far, though, his response to "bittergate" doesn't seem to be working. Jennifer Rubin notes:

If voters are going to forgive and forget they won’t do it because Hillary is being Hillary. Obama’s got to make nice with them. But he seems unwilling to do that because, at bottom, he really doesn’t think he did anything wrong. This is becoming standard operating procedure for Obama. (Associate with a racist; lecture the country on racial unity. Insult a state; lecture us about the mindset of rural Americans.) Lots of lectures and changing the subject; never a full-throated apology.

Wednesday's debate could turn out to be critical.