After announcing his Senate run, Thompson faced three more conservative challengers during a bruising Republican primary election in August. He said the primary left him broke and exhausted.
Baldwin, who ran unopposed in the Democratic primary, seized the opportunity. She and her supporters outspent Thompson 3-to-1 on television advertising in the weeks after the primary, which helped her surge in the polls heading into November.
During her campaign, Baldwin argued that Thompson was not the same man who Wisconsin voters had repeatedly elected to office since 1966. She stressed how he made millions of dollars in the private sector while working for a high-powered Washington law firm and a variety of health companies since 2005.
The criticism stuck.
"Tommy Thompson was popular here as governor, but his activities since then have been questionable," said Baldwin voter Dawn Magnusson, a 39-year pediatric physical therapist from Sun Prairie.
Thompson told voters that Baldwin was too extreme for Wisconsin, noting her support for universal health care and a voting record that ranked her as one of the most liberal members of Congress. Ads against Baldwin, meanwhile, tried to portray her as an extremist and frequently used footage of her from a recent rally shouting, "You're damn right!"
But on Tuesday, it was Baldwin who got the last laugh, knocking down a political legend.
"I am honored, and humbled, and grateful," Baldwin said in her acceptance speech. "And I am ready to get to work."