The Associated Press has declared Tommy Thompson winner of the Republican nomination for U.S. Senate.
The former governor of Wisconsin and the Health and Human Services secretary under ex-President George W. Bush, will challenge Rep. Tammy Baldwin (D-Wis.) in November.
Based on early returns, Thompson was locked in a tight battle with political newcomer Eric Hovde. As returns were being counted, the lead see-sawed back and forth, foreshadowing what was a razor-thin margin.
Republicans haven't held the seat since 1957, but they hope momentum is on their side in the wake of Gov. Scott Walker's recall victory in June and U.S. Rep. Paul Ryan's selection as Mitt Romney's running mate on Saturday.
The primary has been nasty, with the candidates and outside groups spending millions attacking one another while attempting to appeal to the conservative Republican base most likely to show up in a low turnout election.
Turnout was predicted to be 20 percent.
Thompson, 70, had the most name recognition as a governor for 14 years who was first elected in 1966. Thompson hasn't been on the ballot since 1998.