Wisconsin has become an unexpected battleground for Donald Trump and the conglomeration of forces desperately aligning against him, with Tuesday’s primary emerging as a key moment that could reshape the Republican nominating contest both mathematically and psychologically.

Ted Cruz — who has tried to unite conservative activists, talk-radio personalities and the party establishment — stands poised to take some air out of the Trump balloon.

Bleeding from two weeks of self-inflicted wounds and behind in the polls here, Trump scrambled over the weekend to make up ground he has lost to the senator from Texas.

On Sunday, the New York billionaire predicted he would surprise critics. He drew a parallel to his New Hampshire victory in February following a disappointing defeat the week before in the Iowa caucuses — although he was never behind in New Hampshire as he is here.

“We’re having unbelievable response in Wisconsin,” Trump said during a visit to a Milwaukee diner. “And it feels very much like New Hampshire to me, where we started off where, you know, Trump wasn’t going to win New Hampshire, and then all of a sudden, we win in a landslide.”

The Fix's Aaron Blake breaks down what's at stake for the GOP candidates in the April 5 Wisconsin primary. (Peter Stevenson/The Washington Post)

A defeat for Trump would be an embarrassing setback for the front-runner — not just because of the 42 delegates at stake, but because it would demonstrate weakness in a place where he should be strong. The state’s blue-collar demographics, along with party rules allowing independent voters to cast ballots in the primary, have been expected to work in his favor.

A decisive loss also would lessen his chance of amassing the 1,237 delegates needed to secure the nomination outright. Failure to do so would force an open convention in Cleveland in July.

“Wisconsin has always been a barometer state,” said former governor Tommy Thompson, a supporter of Ohio Gov. John Kasich. “What you’re seeing is that The Donald, who has been moving ahead all across the country, has hit a logjam or a brick wall in Wisconsin.”

A fish fry for nearly 1,000 Republican activists inside Milwaukee’s timeworn American Serb Hall on Friday night told the story.

The real estate mogul who has been bulldozing the field did not show up. In his place, Trump sent Sarah Palin, once a deity on the right who on this night was exposed as a mere mortal. As she testified to “the awesome awakening” brought by “the Trump Train,” Republicans in the crowd rolled their eyes. They checked their phones. There were plenty of murmurs, even some laughs.

Palin got it. Wrapping up her speech, she thanked the Wisconsinites for “allowing me to kind of crash your fish fry.”

When Cruz took the stage a few minutes later, the reception was dramatically different. He declared, “Nominating Donald Trump is a train wreck” — and, pausing for effect, added, “That’s actually not fair to train wrecks.” The crowd responded with roaring adulation.

Republican presidential candidate, Texas Sen. Ted Cruz, accompanied by his wife Heidi, left, and Carly Fiorina, speaks at a campaign stop last Wednesday in Madison, Wis. (Andy Manis/AP)

Cruz has sought to exploit Trump’s vulnerabilities with female voters following a string of controversies. The senator staged an event in Madison last week that he called “a celebration of strong women.” Cruz sat in a plush armchair and listened as wife Heidi, mother Eleanor and supporter Carly Fiorina shared stories about him as a loving father, loyal husband and champion for women everywhere.

Campaigning Sunday in Green Bay with a parade of endorsers, Cruz said, “Wisconsin is a battleground. . . . The entire country is looking to this state Tuesday night.”

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Recent polls in Wisconsin show Trump trailing Cruz — in two polls by 10 points, others by single digits — with Kasich running third. The senator has drawn energy, and crucial grass-roots support, after winning the endorsement of Gov. Scott Walker, who is deeply popular among Wisconsin Republicans.

Trump redrew his schedule to devote the final days to barnstorming the state — even missing his new grandson’s bris — in an apparent effort to catch Cruz.

Trump’s campaign has been frustrated in recent weeks as Cruz has seemed to outmaneuver him in some aspects of the delegate race. Cruz’s campaign has begun leveraging arcane party rules to squeeze additional delegates, even in states won by Trump. The front-runner sees a win in Wisconsin as a way to avert a contested convention.

“I really want to win Wisconsin because if we can win Wisconsin we’re going to put all this stupidity away,” Trump said at a rally last week in Janesville.

Charlie Black, a veteran Republican strategist who is now part of Kasich’s team, said of a possible Trump loss in Wisconsin: “I think it’s a big deal because the whole question is can he get to the 1,237. At the rate he’s going, he won’t. I think he’s going to lose Wisconsin and not get very many delegates there.”

What makes Tuesday’s balloting important is that Wisconsin’s electorate plays more to Trump’s strength than to Cruz’s. The percentages of evangelical Christians or Republicans who call themselves “very conservative” are smaller here than in states where Cruz has done best.

Beyond that, Wisconsin’s economy long has had a strong manufacturing base, and Trump has drawn significant support from white, working-class voters with forceful denunciations of free-trade deals that have led corporations to shift jobs overseas.

Cruz’s allies hope a win in Wisconsin could transform the way the Texan’s candidacy is viewed nationally.

“This is a signature win in a blue-collar state . . . that’s outside of the South and the West,” said Keith Gilkes, a longtime Walker adviser. “It demonstrates his ability to coalesce a bigger, broader coalition. That’s the first time he’s done that.”

Former House speaker Newt Gingrich offers a dissenting view about the potential significance of Tuesday’s results, in part because Cruz’s victory is now assumed. “I don’t think much unless the result turns out very, very different than we think it will be,” said Gingrich, who has informally advised Trump. “Cruz should win statewide and half the congressional districts. If he were to sweep as Trump did in South Carolina or Arizona, that would be a bigger thing.”

What also makes Wisconsin important is that it is the only contest on Tuesday. That guarantees outsize attention to the results and to the analysis that follows — the sort of singular focus usually reserved for early-voting states such as Iowa and New Hampshire.

Trump will have to live with the loss longer than in the past, with no opportunity to recuperate until the New York primary April 19. Trump is the heavy favorite in his home state. A CBS News poll released Sunday showed Trump leading in New York with 52 percent, followed by Cruz at 21 percent and Kasich at 20 percent.

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“He will have to show what all winning candidates show, which is resiliency in the face of adversity,” GOP strategist Steve Schmidt said.

Gingrich said that Wisconsin could be a wake-up call for Trump that was badly needed. “It might be good that they’ve had to worry about Wisconsin,” he said.

Cruz’s campaign sees Wisconsin as potential validation that he has emerged as the clear and perhaps only alternative to Trump. “I think that can have a dramatic impact on the race,” Cruz campaign manager Jeff Roe said.

Cruz’s team thinks the candidate could absorb a loss in New York and recover elsewhere in upcoming contests, with a final June showdown in delegate-rich California.

But he will have to fend with Kasich as well as with Trump. Kasich’s team sees the upcoming calendar as more favorable to the Ohio governor than to Cruz. His advisers expect to win a decent number of delegates in New York and are putting significant effort into Pennsylvania, where Kasich grew up. They see opportunities as well in Connecticut, Rhode Island and Delaware.

“The elongated calendar works to our benefit,” said John Weaver, Kasich’s chief strategist. “It doesn’t seem to do so much for Mr. Trump. Probably his advisers would like to go on a cruise with no WiFi.”

Jose A. DelReal in Milwaukee and Sean Sullivan in Green Bay, Wis., contributed to this report.