MADISON, Wis. -

Three of the four Republican U.S. Senate candidates, responding to voter criticism over a negative campaign tone, claimed self-defense as the reason for running TV attack ads.

Businessman Eric Hovde, former Gov. Tommy Thompson, former U.S. Rep. Mark Neumann and their third-party allies have spent millions of dollars on the campaign ahead of Tuesday's primary.

Only state Assembly Speaker Jeff Fitzgerald hasn't run attack ads, partly because of a lack of campaign funding and partly because he said voters are tired of the negativity.

As for which candidate threw the first punch in the TV war, there was only finger-pointing.

"If you are going to besmirch my character and you're going to attack my credibility and my legacy, I'm going to have to defend myself," said Thompson, referring to ads that attacked Hovde and Neumann. "Only then did I get into it and put up any kind of response."

A Thompson ad on the Madison TV airwaves claims "Hovde refused to pay his property taxes until he was taken to court."

WISC-TV has found all of the ads, including that one, to be half-truths at best. For the Reality Check series, visit Channel 3000's Politics section.

Neumann also attacked Hovde through the conservative group Club For Growth. Neumann defended the ad in an interview.

"I think the comparison of policy issues is something that's legitimate," Neumann said. "So putting Eric Hovde on TV saying, 'I'm OK with higher taxes,' for example, that's OK; that's a comparison of policy."

Hovde, in fact, supports broad tax cuts, although he does support eliminating most tax breaks for big businesses and the wealthy.

Hovde began airing an ad that showed him dodging mud his opponents were slinging, although the hedge fund manager then turned to attacks himself.

"I'm not going to sit here and be a complete punching bag," he said, when asked if he took any responsibility for the negative campaigning. "I'm not going to allow everyone to hit on me and not do some type of response."

Fitzgerald, who's in last place in recent polls, has been relying on his record as Assembly speaker, as well as his ties to Gov. Scott Walker.

"I just think we're not focused on the right thing," Fitzgerald said. "I think we should be focused on Tammy Baldwin, who's going to be the Democratic nominee, and to me, I think that's what the race is about."

Recently, the three other candidates appear to be changing course. Hovde is airing an ad in Madison that shows him and his wife making fun of the negative tone, before turning to the issues.

Thompson launched an ad that shows a child holding a small U.S. flag, saying, "I'll fight to keep the American promise for our children and grandchildren."

And Neumann promoted his past in Congress fighting for a balanced budget.