Mr. White lives in the Milwaukee suburbs, the state’s most populous Republican region. Mr. Evers’s gains there last year were a key to his narrow victory. In an interview, Mr. Walker said that should be a lesson to Mr. Trump. “To win, he’s going to have to improve his numbers in the suburbs,” he said. “With the passion the opposition is going to have, he’s going to have to make some inroads.”

Last fall, Mr. Walker matched his vote totals from 2014 in the three suburban counties outside Milwaukee. But a surge of Democratic votes in the demographically changing suburbs meant his vote share dropped in 2018 by 12 percentage points in Waukesha County and by 16 points in Ozaukee County.

“It’s the paradox of Trump’s strategy of appealing to his base constantly,” said Charles Franklin, director of the Marquette Law School Poll of the state. “It also helps gin up the feelings of your opposition. I think that it’s a gift to Democrats that keeps on giving.”

One of those voters is Tiffany Duncan, a 26-year-old employee of a printing company, who skipped the 2016 election because she didn’t like either major party candidate. But she definitely plans to vote in 2020 — for whichever Democrat takes on Mr. Trump. “I didn’t think Trump was this crazy,” she said as she walked to Café Bavaria in Wauwatosa Village.

Still, Mr. Franklin said, the state is so evenly divided, its chances of determining an Electoral College majority for either party “puts us at ground zero.”

In a Marquette poll in January, 49 percent of Wisconsin voters said they would definitely vote for someone other than Mr. Trump. Only 27 percent plan to definitely support the president, meaning his path to a second victory in the state is both uphill and narrow.

Many of today’s uncertain voters — who are Republicans or Republican-leaning independents — may come home to the party, especially once there is a Democratic nominee to contrast with the president.