Democratic congresswoman Tammy Baldwin, 50, would be the first openly gay person elected to the U.S. Senate if she wins election Tuesday. The Madison native would also be Wisconsin's first woman senator.
If voters choose former Gov. Tommy Thompson, 70, it would mean Republicans would occupy both of Wisconsin's Senate seats for the first time in more than half a century.
The winner will replace retiring Sen. Herb Kohl, a Democrat from Milwaukee. The contest has drawn tens of millions of dollars in outside spending, and the outcome could determine whether Democrats continue to control the U.S. Senate or whether they cede power to the Republicans.
On Monday, Baldwin campaigned with President Barack Obama before a crowd of about 18,000 in Downtown Madison. She promised to look out for the needs of average people and to ensure that everyone "plays by the same rules."
Thompson on Monday continued a busy schedule of campaigning in 10 cities over three days, touting his plans for helping turn around the economy.
The candidates have traded leads in the polls since Thompson emerged from a four-way Republican primary in August. However, most surveys predict a tight outcome.