Rubio wavered when he was asked at a campaign event whether he would still support the Republican presidential front-runner if he was the nominee.
"I don't know. I mean, I already talked about the fact that I think Hillary Clinton would be terrible for this country. But the fact that you're even asking me that question, I still at this moment continue to intend to support the Republican nominee, but it's getting harder every day," he said.
That's a break from Rubio's comments in last week's GOP debate, when he said unequivocally he would support Trump because he was a better alternative than the Democrats.
"I will support Donald if he's the Republican nominee and let me tell you why. Because the Democrats have two people left in the race, one of them is a socialist," he said. "America doesn't want to be a socialist country. If you want to be a socialist country then move to a socialist country. The other one is under FBI investigation."
Rubio's Republican rivals have all pledged to support Trump -- a move that some say undermines their attacks against him.
Ohio Gov. John Kasich stopped short of saying he wouldn't stop Trump on Saturday, but he said the "toxic environment" the real estate mogul had created was "makes it extremely difficult" to support him.
Rubio and Kasich both also criticized Trump for violence at recent rallies.
"To see Americans slugging themselves at a political rally deeply disturbed me," Kasich said at a campaign stop in Cincinnati, according to the AP. "We're better than that."
"The protesters are not blameless, some of these people are organized elements who are paid. Others just went there for the intent of disrupting an event. And you don’t have a right to disrupt an event just because you disagree with someone," Rubio said.
"But I think we also have to look at the rhetoric of the front-runner in the presidential campaign. This is a man who in rallies has told his supporters to basically beat up the people who are in the crowd and he’ll pay their legal fees. Someone who’s basically encouraged the people in the audience to rough up anyone who stands up and says something he doesn’t like."
The comments came just days ahead of Tuesday's Florida primary, which Rubio needs to win to stay viable in the presidential race.