Editor's note: This story has been updated.

U.S. Sen. Mike Lee of Utah announced Thursday he is supporting Ted Cruz for president, giving Cruz his first endorsement from a colleague in the upper chamber.

Addressing reporters in Florida, Lee declared Cruz "unstoppable" as the only Republican candidate who can beat billionaire Donald Trump. Lee urged other GOP White House hopefuls to get behind Cruz — particularly Marco Rubio, the only other senator left in the race.

"We've reached a point when we need to unite behind a single candidate," Lee said, speaking hours before the Republican debate in Miami. "Unity in the Republican Party is more important right now than ever before."

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Lee's endorsement gives Cruz a boost five days before the primary in Florida, where Rubio is scrambling to shore up support on his home turf. Lee's backing also arrives 12 days before his home state of Utah holds its caucuses.

Lee's endorsement of Cruz is a blow to Rubio, with whom the Utah senator is also close. Asked why he passed over his colleague from Florida, Lee reiterated his desire for the GOP to coalesce around Cruz against Trump.

"I'm sending the signal that it's time to unite," Lee said, adding that if the Florida senator asked for his input, he "would encourage" him to drop out and get behind Cruz.

In endorsing Cruz, Lee chipped away at the charge that the Texas senator does not get along with his colleagues. That perception has been fueled by Trump and Rubio, who has the backing of 14 Senate Republicans.

Lee predicted the tide would soon turn in Cruz's favor. "I expect that I'll be the first of many Republican senators who will endorse Ted Cruz," Lee told reporters.

Lee has been Cruz's closest ally in the Senate, particularly during Cruz's 2013 efforts to stop the Affordable Care Act that led to a government shutdown. Lee was also an early backer of Cruz in his 2012 Senate run.

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The presidential race began with three GOP senators whom Lee considers friends: Cruz, Rubio and Rand Paul of Kentucky, who dropped out after the Iowa caucuses. In the run-up to the South Carolina, Lee hit the campaign trail for both Cruz and Rubio, leaving open the possibility he could ultimately pick between the two. 

"I've got two really good friends in this race," Lee told reporters before a Cruz event in Easley, South Carolina. "Any one of them running alone would have gotten my endorsement a long time ago."

Addressing supporters a short time later, Cruz called his colleague from Utah "someone I love like a brother."