Updated

Republicans are relying on a veteran Wisconsin lawmaker to make a political comeback and help them take control of the U.S. Senate.

GOP candidate Tommy Thompson was the state governor for 14 years, but until this election cycle he had not been on a ballot since 1998.

The 70-year-old Thompson survived a tough, four-candidate primary Tuesday and now faces seven-term Democratic Rep. Tammy Baldwin, who ran unopposed. They are competing for the open seat of retiring Democratic Sen. Herb Kohl.

Energized by GOP Gov. Scott Walker fending off a recall effort this spring and Wisconsin GOP Rep. Paul Ryan this month being picked as the party’s vice presidential candidate, Republicans think they can win the seat and move closer to taking the Senate majority.

"Wisconsin is on a roll," Thompson said Tuesday after his primary victory.

Republicans need a net gain for four seats to take control of the Senate, but the Kohl seat has been in Democratic hands since 1957, following the death of Republican Sen. Joe McCarthy.

Baldwin, a liberal, and Thompson, who won his first election in 1966, represent a distinct and clearly different choice for voters.

"Voters in Wisconsin will have a clear choice between an extreme liberal from Madison or a proven reformer who can get us working again," Walker said about the race.

Said Baldwin: “The Republican primary electorate presented Wisconsin voters with a clear choice for the November election. Make no mistake, Tommy Thompson will stand with those who already have too much power and influence in Washington.”

An averaging of polls by the website Real Clear Politics has Thompson leading Baldwin by 4 percentage point.

The Rothberg Political Report says most state Republicans think Thompson will be the best candidate against Baldwin, but the Washington-based firm still considers the race a “pure tossup.”

After serving as governor, Thompson left Wisconsin in 2001 to become President George W. Bush's health secretary, a post he held until 2005. He briefly ran for president in 2007 but dropped out due to lack of support.

"Tommy Thompson supports the policies of the past," Baldwin also said.

She was elected to the House in 1998, after serving six years in the state Assembly, and is popular in liberal Madison. Baldwin also has been re-elected with more than 60 percent of the vote every two years since 2002.

But she is far less known in the rest of the state, where the party needs to build support among swing voters. And Baldwin's Democratic agenda will be tough to sell to a divided electorate that has repeatedly rejected many liberal ideals in the aftermath of the recession.

Democrats also are hoping Baldwin to snap a series of losses in Wisconsin that began in 2010, when Republicans took control of the legislature, Walker was elected governor and tea party favorite Ron Johnson knocked off then-Sen. Russ Feingold. Then the conservative-backed candidate for state Supreme Court prevailed last year and Walker fought back a recall effort with a decisive 7-point victory in June.

Thompson also has the support of the three primary candidates he defeated, including Tea Party favorite and former Rep. Mark Neumann

"Tammy Baldwin is somewhere out to the left of Barack Obama in never-never land," Neumann said.

The Associated Press contributed to this report.