Racine County followed a similar path, backing Obama and then supporting Walker in both his initial bid for governor and in the recall.
With the Republican candidates bickering among themselves, Baldwin spent the spring and early summer quietly building her campaign, raising money, meeting with activists and introducing herself to voters. She embraced hot-button topics like health care reform while emphasizing her personal story.
Baldwin, 50, was born to a teenage mother and raised by her grandparents. When she was 9, she was struck with an illness that put her in the hospital for three months. Her grandparents didn't have insurance for her, and Baldwin said they made huge sacrifices to pay her medical bills.
Her upbringing was the focus of her latest television ad, released statewide on Monday. Baldwin uses her own story to help explain her support for Obama's health care overhaul, which requires most Americans to have insurance. She said it makes her angry that her Republican opponents want to "rip it up."
Health care policy also hits close to home for Thompson, who served as Health and Human Services secretary under President George W. Bush. The 70-year-old served 14 years as governor but hasn't been on the ballot in Wisconsin since 1998.
Squaring off with Thompson in the Aug. 14 primary are three more conservative candidates who are trying to tap into the well of tea party support.
Eric Hovde, a millionaire hedge fund manager, has had the highest profile on television. Mark Neumann, a former congressman, has the backing of prominent politicians and conservatives groups. And Jeff Fitzgerald, the speaker of the state Assembly, helped push Walker's agenda through the Legislature.
The most recent poll on the race, conducted in mid-June by the Marquette University Law School, showed Baldwin ahead of or about equal to each of the Republicans except Thompson. He led Baldwin 49 percent to 41 percent — well outside the poll's 4.1 percentage point margin of error.
Conservative groups aren't waiting for the general election to begin attacking Baldwin.
Americans for Prosperity Wisconsin, an anti-tax group, last month launched $400,000 worth of ads against her. That came after the U.S. Chamber of Commerce, in conjunction with the state business group Wisconsin Manufacturers and Commerce, ran a Baldwin attack ad in February.
Baldwin, "in one campaign stop after another, opines on how the middle class is struggling and 'taking it on the chin,'" said state GOP spokesman Nathan Conrad. "Yet she fails to mention that her rubber-stamp approval of any and all policies put forth by the Obama administration have driven middle-class prosperity underground."
But other Republicans, including Thompson and Rep. Jim Sensenbrenner, have warned for months not to underestimate Baldwin.
She laughed when asked why the GOP may be dismissing her chances of winning.
"I have no idea," Baldwin said. "They shouldn't."
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