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Sen. Mike Lee ended his neutrality in the presidential race Thursday and became the first sitting senator to endorse Sen. Ted Cruz, saying his colleague from Texas is the only Republican who could stop Donald Trump and defeat Democrat Hillary Clinton.

"I'm sending the signal that it is time to unite," said Lee, a prominent tea party figure who had promised to back the last man standing between Cruz and Sen. Marco Rubio of Florida. "The time is finally at hand when we, as Republicans, need to unite behind one leader."

The Utah Republican traveled to Florida, skipping votes in the Senate, because he thought it was possible to broker a unity ticket in which Cruz would be the presidential candidate and Rubio his vice presidential pick, according to a source close to the process. When that fizzled, Lee decided to move forward with an endorsement, believing that if both remain in the race, Trump would likely claim the nomination.

Cruz wasn't there to accept Lee's endorsement. Instead, the Utah senator, wearing a white shirt and jacket but no tie, delivered his endorsement alone in front of a smattering of media gathered at the University of Miami in Coral Gables, Fla., the site of a GOP debate hosted by CNN.

Soon after Lee heralded Cruz as a principled conservative and a man of substance, the Texas senator sent out a statement saying: "I couldn't be more humbled by Mike's endorsement. He has been a tremendous friend and I am deeply honored by his support. No one has fought harder to restore the rule of law, end cronyism, and level the playing field for millions of Americans."

Lee's endorsement was not only a boon for Cruz, but also a serious hit for Rubio, coming in his home state just days before Florida's primary Tuesday. Rubio's campaign has been in free fall, having claimed no delegates in the latest contests. Recent polls show him trailing Trump in Florida.

Lee said he had personally urged Rubio to support Cruz before making his endorsement and noted he talks with the Florida senator "on a very regular basis." They became friends when they were both Senate candidates in 2010. When they are in Washington, the two share a Capitol Hill row house with at least one other senator.

That close relationship with Rubio and with Cruz was one of the reasons Lee had declined to endorse up until this point. He even held separate campaign events with each man just before the South Carolina primary.

Lee said he changed his mind Thursday because Cruz has emerged as the clear foil to Trump by winning in states like Idaho on Tuesday, and previously in Kansas, Maine and his home state of Texas, among others.

"He's got a building groundswell of support that is unmistakable," Lee said.

But Cruz hasn't received much love from his Senate colleagues, partly because he has a contentious relationship with many of them. Part of that friction stems from the effort Lee and Cruz led in 2013 to oppose the Affordable Care Act that led to a partial government shutdown for 16 days. Cruz also famously called Senate Majority Leader Mitch McConnell, R-Ky., a liar. Still, Lee said, the party and his colleagues will come around.

"I'll expect that I'll be the first of many Republican senators who will endorse Ted Cruz," he said. "I'm confident more are on the way."

Sen. Orrin Hatch, R-Utah, has endorsed Rubio, as have Utah's four House members.

Twitter: @mattcanham