FactCheck.org analyzes political ads for accuracy:

This year, as in the 2004 campaign, a number of persons have asked us how candidates and other groups can get away with such deceitful advertising. The truth is that the law of libel is not a practical deterrent. There also is no federal “truth in political advertising” law on the books, nor is there likely to be such a law so long as the First Amendment stands. For a full discussion see “False Ads: There Oughtta Be A Law! Or – Maybe Not,” our Special Report from June 3, 2004. Our system of government leaves it to voters to sort out the truth from what they see and hear, with whatever help they can get from a free press.

Perhaps the solution lies in changing the geometry of the question. Instead of trying to regulate the content of ads, we use public funds (or funds from the Parties) to run a frequent public service message that corrects the inaccuracies of any recent ads – perhaps produced by the Factcheck.org folks. Since a candidate would not want to have the airwaves filled with objective criticism they would be inclined to stay as reasonable and civil as possible.

Politics A Remedy for Negative Political Ads