MADISON, Wis. - Republican Tommy Thompson began airing a television ad Monday that accuses Democratic opponent Tammy Baldwin of lying, but an independent fact-checking group found the former Wisconsin governor has been the less-truthful candidate during the U.S. Senate race.

Thompson and Baldwin are battling for the seat that became open with the retirement of Democratic Sen. Herb Kohl. A Marquette University Law School poll released last week showed Baldwin with a narrow 4-point lead.

Both candidates, as well as their allies, have been spending millions on television ads attempting to sway voters. Many of those spots, as well as other statements, mailings and campaign material, are fact-checked by the independent group PolitiFact.

It is that group's work that forms the center of Thompson's latest statewide ad called "Lying." It refers to PolitiFact declaring as "false" comments made in a television spot last month by the Democratic Senatorial Campaign Committee on behalf of Baldwin.

That ad claimed that Thompson "admitted" he had been changed by Washington and had sold out after leaving public office in 2005 to make millions.

PolitiFact determined was false because it mixed and matched Thompson's statements, making it appear he was condemning his own conduct and private-sector work when he wasn't.

Thompson's ad doesn't include that background or make clear it was comments by another group, and not Baldwin, that were determined to be false.

A tally of PolitiFact ratings shows Baldwin has gotten only one "false," while Thompson has gotten six. He's also received three "pants on fire" ratings, which is even worse than a "false."

"Tommy Thompson can't handle the truth," Baldwin's spokesman John Kraus said in a statement reacting to the new ad. "Thompson has turned his desperate campaign into a dishonest campaign based on false claims and outright lies. The only question now is how low he can go on the low road he is taking."

Thompson, in a statement released with the announcement of the new ad, said it is Baldwin who "has been telling a litany of lies throughout this election. When it comes to hard-working middle-class families, my opponent says one thing while her record indicates another."

The "Lying" ad also refers to Baldwin siding with other Democrats in Congress by voting against a 2010 tax compromise. The ad shows footage of President Barack Obama saying that failure to reach a deal would cause taxes to go up $3,000 for the average family.

In addition to the one "false" rating from PolitiFact, Baldwin has received three "mostly false" ratings, two "half true," one "mostly true" and one "true."

Thompson has received three "pants on fire," six "false," one "mostly false," one "half true," five "mostly true" and three "true" ratings.

Of his three "pants on fire" ratings, one of those was for Thompson saying the Wisconsin affiliate of the national conservative group Club for Growth had endorsed his Senate bid. The group does not issue endorsements.

Another was for Thompson claiming in an ad that President Barack Obama's health care law results in a "government takeover of health care." That claim has been repeatedly discredited and was even dubbed the "2010 Lie of the Year" by PolitiFact.

The third "pants on fire" rating was given because of a comment one of Thompson's aides made. The aide claimed that an email circulated by Thompson's spokesman, providing a link to a video of Baldwin dancing at a gay pride event, was not sent on behalf of the campaign. But the spokesman sent it from his campaign account, leading to the "pants on fire" determination.

Baldwin's lone "false" rating was for a campaign email Baldwin sent last month in which she said 7,000 millionaires "gamed the system to pay no income taxes on their fortunes."

PolitiFact determined that was false because the number was too high and she provided no evidence of how those who didn't pay income taxes "gamed the system."