Continued from page 1

The ad is not without its defenders, however. Byron York of National Review reviewed the bill in question and talked with sponsors and concluded that Mr. Obama deserves to be challenged on the education bill because it would, in fact, have required kindergartners to learn about avoiding sexual abuse.

The sole Obama ad to earn “pants-on-fire” was his immigration attack, the Spanish-language ad that accused Mr. McCain of siding with talk-show host Rush Limbaugh, an opponent of the immigration bill. The ad also used quotes out of context to suggest Mr. Limbaugh was insulting immigrants.

John Geer, a professor at Vanderbilt University who studies negative ads, said fact-checking is difficult, and much depends on what facts the checker brings. Mr. Geer pointed to ads in 2004 about the economy and said one could conclude from job numbers that the economy was poor but would conclude from gross domestic product that the economy was booming.

“The problem I have with the fact-checkers is it’s the negative ads that have the facts in them, and they end up only talking about the negative ads. And then, what’s a fact?” he said.

Mr. Geer said it’s not clear this year is the most negative in tone. He pointed to harsh ads in the 1964 campaign and to still harsher charges tossed about in the 1948 campaign as potentially tougher.

“My sense is that what surprises me about this year is there doesn’t seem to be as many positive ads,” he said. “Whether or not they’re nastier, that is unclear.”

He said the press attention to negativity could be a reaction of pundits and reporters looking for an explanation why, with so much of a political tail wind, Mr. Obama does not have a larger lead.

“I’m wondering if in fact some of the reactions are ‘Obama should be up by more, based on all the fundamentals, and he’s not,’” Mr. Geer said. “If Obama was up by 10 points in the polls, would people be worrying about this as much?”