Wisconsin state Assembly Speaker Jeff Fitzgerald (R) on Tuesday officially announced he will run for the Senate, adding another name to the crowded Republican field running to replace retiring Sen. Herb Kohl (D-Wis.).

Fitzgerald, a close ally of Wisconsin Gov. Scott Walker (R), who helped him muscle through a bill to strip public employees of their ability to unionize, will face off against at least two better-known and better-funded candidates. Former Rep. Mark Neumann (R-Wis.) is already in the race, while former Gov. Tommy Thompson (R) has filed paperwork with the Federal Election Commission and is expected to announce his campaign next week.

The union issue was the motivation for his campaign, Fitgerald told The Associated Press.

Its the No. 1 thing Ive got going for me. Its fresh in peoples minds, he said. We did it right in Wisconsin. ... There are conservatives who think Wisconsin is ground zero for the movement.

Both Neumann and Fitzgerald are to the right of Thompson on policy. If Fitzgerald can get any traction he could complicate Neumann's bid to emerge as the conservative alternative to Thompson. National conservatives including Sen. Jim DeMint (R-S.C.) and the deep-pocketed Club for Growth have already endorsed Neumann and slammed Thompson for raising taxes while governor and backing early versions of President Obama's health care law.

Neumann ran a sometimes-contentious primary race against Walker last election. If Walker, who is very popular with the conservative base and is facing the potential for a recall election next spring, decides to get behind Fitzgerald, the primary could be further complicated.

The only Democrat in the race is Rep. Tammy Baldwin (D-Wis.), a liberal who would be the first openly gay senator in U.S. history.

Neumann welcomed Fitzgerald to the race and attacked Baldwin in a news releasee.

I respect Speaker Fitzgerald and his dozen years of work in the state legislature, Neumann said. But my campaign is focused on solving America’s debt crisis, balancing the budget and repealing Obamacare,. This race is about choices. Tammy Baldwin is the most liberal member of Congress. I’m Wisconsin’s most conservative congressman in the last 30 years. It’s as simple as that. I’m convinced that'’s why the Club for Growth has endorsed me.