JANESVILLE, Wis. — The danger signs are mounting for Donald J. Trump in Wisconsin: Right-wing radio hosts are flaying him, Gov. Scott Walker and other elected Republicans have endorsed Senator Ted Cruz, and a new poll showed Mr. Cruz with a 10 percentage-point lead in the state before Tuesday’s primary.
The Stop Trump movement may never have another opportunity like the one here, where resistance to Mr. Trump was running high even before his campaign became consumed by a new round of controversies, from his mocking of Mr. Cruz’s wife to the arrest of his campaign manager to his comments in favor of punishing women who get abortions.
If Mr. Trump is dealt a setback in the Wisconsin primary, including a potential sweep by Mr. Cruz of all 42 delegates, it would be his most prominent reversal since his second-place finish in the Iowa caucuses in February. And it would show Mr. Trump’s vulnerability before the race moves to New York and other Northeastern states.
The state’s Republican establishment, cohesive and battle-tested after years of partisan warfare under Mr. Walker, has dug in to support Mr. Cruz — not out of true love for the Texas senator, but in a marriage of convenience to halt Mr. Trump, whose temperament and conservatism many doubt.
But at the same time, if the forces arrayed against Mr. Trump do not prevail in Wisconsin, they are unlikely to slow Mr. Trump in the ideologically more favorable turf of the East, increasing his chances of locking down the nomination before the July national convention.
A poll released Wednesday by Marquette University Law School showed Mr. Cruz leading the Republican field with 40 percent and Mr. Trump with 30 percent, a reversal from a month earlier when Mr. Trump held a 10-point lead. Gov. John Kasich of Ohio was in third, with 21 percent.
A Cruz victory would suggest that a backlash against Mr. Trump has set in after a series of nasty episodes, including his insults of Heidi Cruz and the arrest of Corey Lewandowski, Mr. Trump’s campaign manager, on a charge of manhandling a female reporter.
Mr. Cruz went further than ever before on Tuesday during a CNN town hall in Milwaukee, where he suggested he would not support Mr. Trump if he became the party’s nominee.
“It’s gotten really ugly,” Mr. Cruz said. “What lesson do our kids take watching us?”
Earlier he said the battery charge filed in Florida against Mr. Lewandowski should be “a fire-able offense.”
With Mr. Trump adamantly defending Mr. Lewandowski as the victim in the encounter, after video evidence contradicted the manager’s earlier charge that the reporter was “delusional,” Mr. Cruz said, “Nominating Donald Trump would be an absolute train wreck.”
Mr. Trump’s unfavorable rating in the Marquette poll, conducted before Mr. Lewandowski’s arrest, was 70 percent. Only 24 percent of women planning to vote in the Republican primary backed Mr. Trump, fewer than for the other two candidates.
Still, Mr. Trump has many advantages in Wisconsin, including its large number of white working-class voters, a group that has flocked to him throughout the campaign, and a passionate base of supporters for whom he can do no wrong. He plans a heavy schedule of appearances through the weekend.
Thousands waited more than two hours on Tuesday to hear him in Janesville during the candidate’s first visit to the state. At the rally, he belittled Mr. Walker for his record and even his love for Harley-Davidson motorcycles. “He doesn’t look like a motorcycle guy to me, I’m sorry,” Mr. Trump said to laughter.
Neither women nor men waiting to hear him, many of whom bought Trump hats, buttons and shirts from vendors working the crowd like a Green Bay Packers game, said they were unsettled by Mr. Trump’s derogatory remarks about women.
“I think he calls ‘em as he sees ‘em,” said Mae Pospeschil, a title searcher from Beloit.
Unlike in neighboring Illinois and Michigan, Midwestern industrial states where Mr. Trump had strong victories, the race in Wisconsin is much tighter, in part because of sharp regional differences among Republicans.
Voters in the Milwaukee suburbs, the reddest counties in the state, hold highly negative views of Mr. Trump, according to a polling analysis by The Milwaukee Journal-Sentinel. Voters in northern and western counties are more slightly favorable to Mr. Trump, the Marquette poll showed.
Ed Goeas, a pollster who works for a “super PAC” opposed to Mr. Trump, which is hammering him with television ads in Wisconsin for insulting women, said married Republican women were turning against the New York businessman.
“I see an opportunity for us to have Trump walk out of there with no delegates,” Mr. Goeas said. With no Republican nominating contests for two weeks after Wisconsin votes, a Trump defeat could change the tenor of the race.
“The narrative that comes out of Wisconsin has a huge impact,” Mr. Goeas said.
Mr. Trump is struggling in the suburbs with its more affluent and better educated voters, the heart of Walker territory, which sustained the governor in a bitter recall election in 2012 and his re-election in 2014. “The Republican base still loves Scott Walker,” said Tom Schreibel, a former chief of staff for Representative Jim Sensenbrenner, who represents the region. “The further you get from major metropolitan areas, Trump gets stronger.”
Another factor in the race is the influence of the state’s right-wing talk radio, which has been consistently anti-Trump.
Charlie Sykes, a radio host in Milwaukee, who derisively calls Trump supporters “Trumpkins” for their unquestioning loyalty, subjected Mr. Trump to a punishing interview this week. “Remember, we’re not on the playground,” he chided the businessman over his insults of Mrs. Cruz. “We’re running for president of the United States.”
The regional differences matter because of how Wisconsin awards delegates. Three each go to the winner of each of the state’s eight congressional districts. Eighteen more are awarded to the winner of the statewide vote.
Mr. Cruz has brought his most intensive retail effort to the state since the early nominating contests in Iowa and South Carolina, campaigning in Wisconsin for a week and setting up a “Camp Cruz” for volunteer door knockers in the Milwaukee suburbs.
“Wisconsin will be a solid example of our campaign’s ability to run a single-state race, much like Iowa,” which Mr. Cruz won, his campaign pollster, Chris Wilson, said in an interview.
But Mr. Cruz’s efforts are threatened by Mr. Kasich. The Cruz campaign says Mr. Kasich will siphon off anti-Trump votes and hand Mr. Trump the three delegates from some districts.
Recognizing the threat, a pro-Cruz “super PAC” began a harsh anti-Kasich radio ad in the state on Tuesday.
Mr. Kasich’s campaign vehemently rejects the charge that he is a spoiler. It maintains that he is strong enough to win two or three districts in Wisconsin: around Madison, the liberal state capital; and the close-in Milwaukee suburbs, where Mr. Kasich campaigned on Tuesday. The Marquette poll showed Mr. Kasich leading in the Madison media market.
“I think that if Kasich is not in, Donald would win altogether more delegates than with Kasich involved,” said Tommy Thompson, the former Wisconsin governor, who is chairman of Mr. Kasich’s campaign in the state. Like other Kasich supporters, he is banking on the Ohio governor emerging as the consensus nominee in an open convention as the best choice to defeat a Democrat in November.
“It’s short game versus long game,” he said.