The deep-pocketed, fiscally conservative Club for Growth endorsed former Rep. Mark Neumann's (R-Wis.) Senate campaign Thursday morning.

"The Club for Growth PAC proudly endorses Mark Neumann for the United States Senate," said Club for Growth President Chris Chocola. "In Congress, he fought members of his own party on spending, received straight As from the National Taxpayers Union, and was a leader in the fight for limited government and economic freedom. Club members and Wisconsin Republicans looking for a fiscal conservative and pro-growth champion to send to Washington have a perfect candidate in Mark Neumann."

The group has been on the warpath against former Wisconsin Gov. Tommy Thompson (R), who is likely to announce his own campaign. Thompson was an early supporter of President Obama's push to reform health insurance, although he did not support the final bill. The group has already run ads against Thompson, criticizing him on healthcare and his tax record.

Neumann ran for governor last year, losing to now-Gov. Scott Walker (R) in the primary by a 20-point margin. Some of his former staffers, including his former chief of staff, now work for the Club for Growth. Thompson's campaign has stressed those ties in an attempt to discredit the group and attack their likely opponent.

"It's widely known that Mark Neumann’s former employees work at the Club for Growth," said Thompson spokesman Darrin Schmitz. "Given Neumann's record of distortions regarding Governor Walker, we’re not surprised he's now enlisting his former employees to do his dirty work."

Polling shows that Thompson leads Neumann, but not by an overwhelming margin. Both a poll conducted by the Club for Growth and one conducted by Democratic-leaning Public Policy Polling give him just single-digit leads against Neumann. Both polls then told respondents that Thompson supported Obama's health insurance reform law, a message Neumann is sure to push, and retested the matchup. In both cases, Neumann then jumped to a strong lead over Thompson.

State Assembly Speaker Jeff Fitzgerald, a conservative Republican who helped Walker push through a law stripping state employees of unionization rights, is also in the race. Fitzgerald could play spoiler — if he gains enough traction he and Neumann could split the conservative, anti-Thompson vote and give the former governor an easier road to victory.

The seat is currently held by Sen. Herb Kohl (D-Wis.), who is retiring. Democrats mention Reps. Tammy Baldwin and Ron Kind and former Rep. Steve Kagen as possible candidates.

—This post was updated at 10:43 a.m.