Wisconsin’s Republican primary is shaping up as a fateful test of whether Sen. Ted Cruz can blunt the momentum of Donald Trump, and anti-Trump forces are rushing to back Mr. Cruz, the GOP establishment’s unlikely champion.

With its deep reservoir of committed conservative voters, Wisconsin, which votes in two weeks, is considered fertile ground for Mr. Cruz, who’s also seeing millions of dollars in spending by his allies and anti-Trump political action committees.

“It’s Ted Cruz’s to lose,” said Brian Fraley, a Republican political strategist in Wisconsin. “The never-Trump movement is pretty strong here.”

Wisconsin awards 18 delegates on a winner-take-all basis to the top statewide vote-getter and a further three delegates to the winner in each of its eight congressional districts. That’s a potentially significant haul in Mr. Trump’s hunt for the 1,237 delegates needed to clinch the nomination and avoid a contested convention.

Mr. Fraley said the state’s conservatives were in “fighting shape” after five years of battling for Gov. Scott Walker, a tea party darling who survived a recall election and implemented a conservative agenda, including winning a showdown with state workers unions.

Those conservative voters are primed to back Mr. Cruz, spurred on by local talk radio hosts who are fervently leading the never-Trump movement in the state. But Mr. Fraley warned that it wouldn’t be a slam dunk and Mr. Cruz still must woo supporters, saying the Texan “needs to move to Wisconsin for the next two weeks.”

Indeed, Mr. Cruz campaigned in the state Wednesday and Thursday, and had three stops planned for Friday.

Mr. Trump so far only has a rally scheduled Tuesday in Janesville. The Trump campaign did not respond to questions about its plans for the state.

The stakes couldn’t be higher in the Badger State, said GOP campaign consultant Michael McKenna.

It could be Mr. Cruz’s best chance to flip the momentum, he said, adding that a Trump victory in Wisconsin would set the stage for the front-runner to sweep upcoming delegate-rich contests in New York, New Jersey and Pennsylvania.

“If Trump beats him in Wisconsin, a lot of air comes out of everyone’s balloon and the question becomes how far are they willing to go to change the rules [at the GOP convention] to prevent this guy from being the nominee,” he said.

The race in Wisconsin was extremely close, according to an Emerson College poll released this week in which Mr. Cruz barely edges out Mr. Trump, by 36 percent to 35 percent, though that 1-point lead is well within the poll’s 4.6 percentage-point error margin. Ohio Gov. John Kasich trailed at 19 percent in the survey of likely GOP voters.

Mr. Cruz could be getting a timely boost from the Republican governor. Mr. Walker, who ended his presidential bid early in the race, hinted this week that he could make an endorsement before the state’s April 5 primary and that it probably won’t be for Mr. Trump.

“I think it’s fair to say that my views, my beliefs, my strategy overall would probably be more aligned with either Sen. Ted Cruz or Gov. John Kasich,” Mr. Walker told radio host Charlie Sykes, a “Never Trump” adherent.

“I think if you’re looking at the numbers objectively, Ted Cruz — Sen. Cruz is the only one who’s got a chance other than Donald Trump to win the nomination,” said Mr. Walker. “Statistically, my friend Gov. Kasich cannot.”

The state soon will be flooded with anti-Trump advertising.

Our Principles PAC was cueing up an onslaught of on TV, radio, digital and direct mail to hit Mr. Trump for past liberal positions, his harsh remarks about women, and accusations that his businesses, such as the Trump University real estate school, ripped off people.

Tim Miller, the group’s spokesman, said Wisconsin wasn’t a must-win for the anti-Trump campaign but it was a place where Mr. Trump was vulnerable.

“At this point it is a delegate battle so no single state is a must-win but it is not a strong state for Trump despite this mistaken notion that he will do better in the rust belt/Midwest,” he said.

He noted that if Mr. Trump lost in Wisconsin, he would have “a long drought” of winning only a single state from March 22 until April 19, when Mr. Trump’s home state of New York holds a primary.

The Club for Growth, an anti-tax group that is backing Mr. Cruz, plans to spend between $1.5 million and $2 million on ads in Wisconsin.

Its first TV ad is scheduled to hit the airwaves statewide Saturday. The 30-second spot titled “Math” makes the case that only Mr. Cruz can beat Mr. Trump, and a vote for Mr. Kasich helps Mr. Trump.

“It’s time to put differences aside,” the announcer says in the ad. “To stop Trump, vote for Cruz.”

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