John McCain's new television ad suggests that Barack Obama didn't pick Hillary Clinton as his running mate because he couldn't handle her criticisms--i.e., "the turth hurt and he couldn't handle it." It's a dubious assertion, but the merits of the argument are, for all practical purposes, besides the point. This ad is designed to stoke the resentment of wavering Clinton voters and to make sure the Clinton controversy remains part of the convention storyline. I'll leave it to others to determine whether this gambit will work. I wouldn't think so, but, then, I didn't think this whole saga would last as long as it has.

Clinton has already released a response:

Hillary Clinton's support of Barack Obama is clear. She has said repeatedly that Barack Obama and she share a commitment to changing the direction of the country, getting us out of Iraq, and expanding access to health care. John McCain doesn't. It's interesting how those remarks didn't make it into his ad.

That's fine, although a visual--say, Clinton offering a sharp, emphatic response for the cameras--might help a lot more, particularly if it's quick. That way, it would refute the McCain argument while it's still in the news, rather than prolonging the story longer than necessary.

In the long run, though, what really matters is how the convention unfolds. If Clinton's supporters get the message that Obama believes what they do on the issues--and, perhaps more important, if that get that message from Clinton herself, in her convention speech--then they will come around.

Clinton has certainly shown herself capable of making that kind of appeal. Just think back to her final farwell speech, which was one of the high points of the post-primary campaign for Obama. But she'll need to do it again.

Having said all that, the media has some responsibilty here, as well. Controversy makes for good coverage, I know. But for all the talk of disunity, the really remarkable story about the Democrats right now is the absence of meaningful dissent on the party's agenda. When it comes to substance, the Democrats are arguably more united than they have been since the early 1960s. Yes, you can find divisions on both domestic and foreign policy, on everything from the relative priority of deficit reduction to America's response to Darfur. But these debates don't match the kind we've seen in the past.

--Jonathan Cohn