"I'll cross that bridge when I get there, but we'll lose," he said when asked if he would vote for Trump as GOP nominee.

"No, we won't lose," Graham added. "We'll get slaughtered."

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Graham's comments come after Trump carried more than 45 percent of the vote in Tuesday night's Nevada Republican caucuses.

Asked about Trump's win, the South Carolina senator, himself a presidential contender this election cycle, said he thinks the real estate mogul "has momentum," and that Republicans have until early March to try to stop him from becoming the nominee.

Graham has been one of Trump's most vocal critics, both during his own bid and as a surrogate for former Florida Gov. Jen Bush, who recently ended his White House run.

While Trump has yet to pick up a single Senate endorsement and many vulnerable lawmakers are wary of tying themselves to the GOP front-runner, Republican senators have publicly stuck with the strategy that they'll support the eventual nominee.

Rubio came in second place in the Nevada caucuses behind Trump.