Retiring U.S. Sen. Herb Kohl urged people to vote, predicting that the presidential election would be close.
"The election will be decided by just a few votes, not millions of votes," he said.
Ryan, meanwhile was returning home for a night rally at the Milwaukee airport after a day of campaigning in other battleground states. Ryan has been a frequent visitor to Wisconsin since getting added to Romney's ticket in August, but Romney's stop near Milwaukee on Friday was his first trip to the state since he picked his running mate.
A number of Romney surrogates, including Louisiana Gov. Bobby Jindal, were campaigning for him in Wisconsin on Monday.
Baldwin, in her comments at the rally, said the election offers a choice between two visions for the country.
"Your voices will be heard because Wisconsin, we need a senator and a president who will fight for us," she said to cheers from the hometown crowd.
The Senate race is the most expensive for a Senate seat in state history, with spending exceeding $65 million and climbing, and it's been marked by a barrage of negative ads branding Baldwin as a screaming, extremist liberal and Thompson as an uncaring millionaire who abandoned his Wisconsin roots.
Baldwin, 50, gave up her safe congressional seat to run for the Senate after Kohl announced his retirement. Baldwin is vying to become Wisconsin's first woman senator and the first openly gay candidate elected to the Senate. Thompson, 70, is trying for a political comeback after serving as governor for 14 years. Briefly a candidate for president in 2007, and U.S. health secretary for four years, Thompson hasn't been on the ballot in Wisconsin since 1998.
The most recent Marquette University Law School poll released last week showed Obama leading Romney by 8 percentage points in Wisconsin among likely voters and Baldwin leading Thompson by 4 percentage points, with a 2.8-point margin of error.
Thompson told WTMJ-AM radio in Milwaukee on Monday morning he felt optimistic, saying he's been traveling all over the state.
"We put on 1,200 miles in the last three days. The crowds were great. The excitement is there. And the polls don't pick up the kind of enthusiasm that our base has ... and that's going to make the difference tomorrow. I feel very good about where I'm at. I feel very good about Romney and Ryan."
Associated Press writers Todd Richmond in Madison and Gretchen Ehlke in Milwaukee contributed to this report.