Republican presidential candidate Ted Cruz received his first endorsement from a Senate colleague when Sen. Mike Lee of Utah backed him for the party's nomination. (Reuters)

CORAL GABLES, Fla. -- Sen. Mike Lee (R-Utah) endorsed Sen. Ted Cruz (R-Tex.) on Thursday, becoming the first sitting senator to back his presidential candidacy.

Lee did so on the home turf of Sen. Marco Rubio (R-Fla.), with whom Lee is close.

"I’m sending the signal that it’s time to unite," Lee said when asked what type of signal his endorsement of Cruz sends to Rubio. "The time has finally come, the time is finally at hand where we as Republicans need to unite behind one leader."

When asked if Rubio should drop out, Lee said: "Everyone will have to make that choice. If Senator Rubio were asking me that, I would encourage him, and I do encourage him to get behind Ted Cruz."

Lee waited months to make his endorsement, and said Thursday was the right time.

"I was waiting, looking for the right moment when it was time to unite," he said.

When asked if he considers Donald Trump a conservative, Lee said he doesn't "think I can make that determination based on the information that he’s given us so far."

Cruz is not well-liked among his Senate co-workers, but Lee is an exception. Lee backed a Cruz-sponsored bill on domestic surveillance last year; Rubio has said during the course of the presidential campaign that the bill endangered national security, and Lee has defended Cruz.

But Cruz and Lee have broken ranks. Cruz said that a criminal justice reform bill Lee sponsored could have dire consequences and release violent offenders from prison.

Lee's office and Cruz's campaign declined comment.

Kelsey Snell contributed to this report.