Democratic candidates, meanwhile, have reacted to similar challenges by ignoring them as best they can. The Obama campaign failed to respond when The Washington Post cited data from the U.S. Census and Bureau of Justice Statistics to challenge the candidate's statistics on the number of young black men in prison and college at an NAACP forum in July. The campaign made no effort to provide supporting data when Obama repeated the claim on November 29 at a fund-raiser in Harlem.

Obama campaign officials declined to comment Saturday.

Many campaign fibs fall into the category of half-truths. Highly selective representation of the facts has become the staple of politics. By using data selectively and playing with language, candidates can reach diametrically opposite conclusions.

According to Giuliani, for instance, taxes went up in Massachusetts while Mitt Romney was governor. Romney insists that they went down. Romney claims that New York City spending increased during the eight years that Giuliani was mayor. Giuliani says that it decreased.

Sorting out who is telling the truth is often a matter of getting the candidates to be more precise. Giuliani's claim that spending declined in New York City refers to per capita spending rather than overall spending. When Romney says that he lowered taxes in Massachusetts, he is referring to three or four very narrow tax cuts. He raised corporate taxes and various miscellaneous state fees.

Setting the record straight on such matters takes time and effort. Days, sometimes weeks, can pass before a falsehood is rectified and it may be difficult to change voters' initial impressions. "It is easy to get the spin out, but it takes longer to get the facts out," said Gehrke, of the DNC.

Jamieson, the University of Pennsylvania professor, believes that candidates are being held accountable quicker than ever. She cites the example of the 2000 campaign when Al Gore misrepresented the position of his Democratic rival Bill Bradley on flood relief for farmers two weeks before the Iowa caucus. The first substantive media critiques of Gore for exaggeration did not appear until after Bradley was roundly defeated in Iowa, and his candidacy was pretty much crippled.

The process of spotting and correcting mistakes can still vary greatly. The Boston Herald quoted Romney as saying that "my father and I marched with King" as far back as 1978. The most recent reiteration of the quote was on Dec. 6, but it was not until Dec. 21 that Romney's description of the episode was first challenged by the Boston Phoenix.

Giuliani had been making his prostate cancer and "socialized medicine" claim for weeks on the campaign trail without being challenged. It was not until he turned it into a radio advertisement on Oct. 29 that several media organizations, including The Post, began examining the claim more closely.