By Brendan O'Brien
MADISON, Wis (Reuters) - Former Wisconsin governor Tommy Thompson officially launched his campaign Thursday for the Senate seat being vacated by four-term Democratic Sen. Herb Kohl.
Thompson, a Republican, served four terms as governor and then as Secretary of Health and Human Services in the George W. Bush administration. He joins a field for the Senate seat that currently includes Congresswoman Tammy Baldwin, a Democrat, and Republican Mark Neumann, a former congressman.
Wisconsin Assembly Speaker Jeff Fitzgerald, a Republican, is also vying for the seat. Fitzgerald was instrumental in passing controversial collective bargaining reform legislation in Wisconsin earlier this year.
Kohl, who has served since 1989, announced his retirement in May. Democrats in 2010 lost a long-time Senate seat held by the party when former Wisconsin Democratic Senator Russ Feingold was beaten by Republican challenger Ron Johnson. Feingold has decided not to run for any office in 2012.
Wisconsin is expected to be a major battleground in 2012, when Democratic President Barack Obama runs for reelection. If Republicans gain three seats in the 2012 Senate elections, the party could take over majority control of the upper chamber.
(Writing and reporting by Brendan O'Brien; Editing by Mary Wisniewski)