According to David Kurtz at Talking Points Memo, the following is a negative ad:
Both Democratic candidates were invited to a televised debate here in Wisconsin. Hillary Clinton has said yes. Barack Obama hasn't. Maybe he'd prefer to give speeches than have to answer questions. Like why Hillary Clinton has the only health care plan that covers every American. And the only economic plan that freezes foreclosures. Wisconsin deserves to hear both candidates debate the issues that matter. And that's not debatable.The negativity, I suppose, would be said to derive from tone of voice during the suggestion that Barack would rather give speeches than debate, and perhaps from that suggestion itself. Except it's true, isn't it? So we're down to a slight change in tone of voice making an ad "negative"?
If we're truly to the point where it is "negative" to draw any distinction between yourself and your opponent, and to suggest that your approach is better, we've stripped the term of all meaning.
This seems to be an unfortunate example of a mainstream media-driven theme "Mean, evil Hillary" spilling over into a media site that usually does better. Obama doesn't drive that media negativity, but he surely does benefit from it.
At the end of the day, if Dick Morris is correct, Obama doesn't have much to worry about.