As Republican U.S. Senate candidates Tommy Thompson and Eric Hovde pull to the front of the field, a conservative political action committee Friday launched an ad campaign attacking the two frontrunners.

The Washington, D.C.-based Club For Growth PAC, which has endorsed Mark Neumann in the Senate race, is airing a 30-second ad that portrays Thompson as a tax-happy former governor and says that Hovde is "Only Worse," the ad's namesake.

According to the TV spot, Thompson advocated nine tax hikes in his time as Wisconsin governor, and Hovde "supported tax-raising governor Jim Doyle."

This ad campaign comes days after a new Marquette University poll shows Thompson and Hovde at the top of the four-man field for the Aug. 14 primary and Neumann being backed with 10 percent of those survey.

After the poll was released, Neumann sent a blunt email to supporters, saying "If the election were held today, we'd lose."

• Related: Watch the Club for Growth ad here

In a pre-emptive email to supporters late Thursday night, Thompson touted his job-creating record as governor and campaign goals. He also turned the tables on the PAC, calling its "reputation" into question:

"The National Club for Growth is also a group that was fined $350,000 for their underhanded campaign tactics," he said in the email. "The National Club for Growth has a reputation, and it's not one to be proud of."

In a press release titled "Neumann's Attack Dogs Go Negative" Friday morning, the Thompson campaign again lashed out at Club for Growth, claiming that Thompson cut taxes 91 times.

The release quoted Darrin Schmitz, a consultant for the Thompson campaign:

Mark Neumann should be ashamed of himself. He ran an ugly and deceitful campaign against Scott Walker and now his attack dogs are grossly distorting Tommy Thompson's record. Tommy Thompson cut taxes 91 times during his tenure including three income tax cuts, the largest property tax cut in the history of the state and he eliminated the inheritance tax. Wisconsin's overall tax burden went down and the state experienced record job growth due to Thompson aggressive tax cuts.

The Hovde campaign responded in a similar fashion, and even though Club for Growth sponsored the ad, Hovde's side called out Neumann for dirty campaign tactics and going after Thompson as well:

Mark ran a negative and nasty campaign against Governor Scott Walker in 2010, and he's at it again now that he's polling in the low-teens and single digits in Wisconsin's U.S. Senate race. ... Tommy Thompson is a career politician and corporate lobbyist who increased state spending by 118 percent, raised taxes and fees by millions of dollars and increased our debt by $2 billion in just five years as governor. Then, Governor Thompson got rich quick as a corporate lobbyist by profiting off his time in office.

In a statement to Patch Friday, Chip Englander, campaign manager for the Neumann campaign, said the attack ads were not a part of the Neumann campaign.

"We are legally prohibited from communicating with the Club for Growth," Englander said. This legal provision was part of the Citizens United v. FEC Supreme Court decision. "We were unaware of the ad's content before we saw it on TV."

Englander added that Neumann is the "proven conservative in the race."

A Club for Growth spokesperson told The Hill Friday that the attacks can be expected for a while, calling their ad purchase "massive."

"Club members believe in supporting principled, limited-government candidates. We don't have any problem with backing underdogs," the spokesperson said.

The Hill also reported that the group has not disclosed how much it spent on the ad campaign.

Check back at Patch.com for updates on the U.S. Senate race.