Republican presidential candidate Ted Cruz praised Wisconsin Gov. Scott Walker at a rally in Brookfield, Wis. Earlier that morning, Walker announced he was endorsing Cruz. (Reuters)

BROOKFIELD, Wis. -- Wisconsin Gov. Scott Walker endorsed Ted Cruz for president on Tuesday, throwing his support to the Texas senator a week before his state's crucial Republican presidential primary and becoming the latest prominent Republican to line up against Donald Trump.

Walker announced his decision during an interview on WTMJ radio with host Charlie Sykes, a Cruz supporter.

"After a lot of thought, a lot of time, a lot of prayer about this, I just really decided -- after all these years of the Obama-Clinton failures -- that it's time that we elect a strong new leader," said Walker. "I just fundamentally believe that he is a constitutional conservative. Why that's important to me: I know, that as a governor, we need a president who understands that our founders intended for the power really to be in the states and in the hands of the people, not concentrated in Washington."

Walker launched his own run for president last year. But after struggling to gain ground, he ended his bid in the fall. He joins a growing roster of mainstream Republicans rallying around Cruz, who has irked the Republican establishment throughout his Senate tenure. Sen. Lindsey O. Graham (R-S.C.) and former Florida governor Jeb Bush, two ex-candidates and fierce Cruz critics, have set aside their differences with Cruz to support him in the hopes that he will defeat Trump.

At Cruz's first event of the day in Brookfield, Wis., his campaign played Walker's interview live on loudspeakers. The crowd applauded when Walker announced his decision.

"How's that for some breaking news this morning," said Cruz when he took the stage. He called Walker a "strong, principled conservative."

Trump tried to undercut Walker's endorsement before he announced it.

"After the way I beat Gov. Scott Walker (and Jeb, Rand, Marco and all others) in the Presidential Primaries, no way he would ever endorse me!" Trump tweeted Monday.

Former Wisconsin governor Tommy Thompson, who has endorsed Gov. John Kasich (R-Ohio) for president, was similarly dismissive of the expected Walker endorsement in an interview last week.

"I don’t know what’s so surprising about it," said Thompson. "I’m sure Ted Cruz is going to help him pay off his campaign debt, and I think he wants to be a player in Ted Cruz’s campaign. I can understand where the governor is coming from. I don’t think it’s going to mean much."

Cruz, Trump, and Kasich are all campaigning in Wisconsin on Tuesday. Cruz is holding at least two events before a CNN town hall that will separately feature all three candidates. Trump is holding at least one event before the town hall. Walker, who called into Sykes's show from more than 100 miles away from Cruz's event Tuesday, said he would campaign with the senator as soon as the logistics were finalized.

Stumping in Wisconsin on Monday, Cruz challenged Trump to meet him in a one-on-one debate. On Twitter, Trump swatted down the idea.

"Lyin'Ted Cruz is weak & losing big, so now he wants to debate again. But, according to Drudge,Time and on-line polls, I have won all debates," Trump tweeted.

Wisconsin will be the center of the Republican political universe for the next week. It is the only state holding a nominating contest next Tuesday. For Cruz, and more broadly for the anti-Trump forces in the Republican Party, Wisconsin is shaping up as a must-win state.

There has been little public polling on the race. The Cruz campaign is anticipating a close contest, and sees the Milwaukee suburbs -- where talk radio hosts like Sykes have pummeled Trump for months, and where his negative ratings are higher than in any comparable region of the Midwest -- as the base for a narrative-shifting win. But while Sykes has always described his support for Cruz as a somewhat grudging #NeverTrump decision, Walker made a positive case for backing Cruz.

"I wanted to make sure I was for someone, not just against someone," said Walker. "He is a decent man. He loves his family. He loves his wife. He adores his children. He and I are both preachers' kids."

Sykes asked Walker several questions about Trump's tone and judgment; for the most part, the governor demurred and turned the conversation back to Cruz's strengths. He blamed "the media" for last week's Trump-driven spat about the wives of the two front-running candidates. He argued "we'd be living under amnesty right now if not for Ted Cruz," praise of the Texan's Senate career that no rivals had ever given.

Walker criticized Trump only after Sykes asked what he thought of the candidate's criticism of his jobs record in Wisconsin.

"Those are the talking points, unfortunately, of the left," said Walker. "They haven’t penetrated in three elections."

Trump donated to Walker's successful 2012 campaign to beat a recall vote, and donated even more to the Republican Governors Association. But in the summer of 2015, as a rival in the presidential primary, Trump heaped scorn on Walker for what he said was a flimsy jobs record and "disastrous" budget management. While Walker's approval numbers have slipped since his 2014 reelection, he remains phenomenally popular with Republicans in Wisconsin. Trump's foes here are watching his first campaign stop, an afternoon rally in Janesville, to see if he mocks Walker again and courts a backlash.