"These young Americans have the chance to fulfill an incredible potential and destiny, but we have to give them a chance, and they won't have a chance if a Hillary Clinton or Bernie Sanders is elected," he said at the Conservative Political Action Conference in National Harbor, Maryland. "And they won't have a chance if the conservative movement is hijacked by someone who is not a conservative."
The audience roared with approval and responded with a prolonged standing ovation.
"Being a conservative can never be about simply an attitude," Rubio also said in the speech. "Being a conservative cannot simply be about how long you're willing to scream, how angry you're willing to be, or how many names you're willing to call people. That is not conservatism."
Rubio, however, did not mention Trump by name.
Pressed by CNN's Dana Bash after his address, Rubio again alluded to Trump.
"The American dream isn't about how much money you make or how many buildings have your name on them," Rubio told Bash. "I wasn't talking about anyone in particular. I was just saying."
"Now I know you weren't talking about any particular person when you just said that," Bash responded, sarcastically.
"I probably was," Rubio conceded, to which the audience laughed. "I was just trying to be nice."
Rubio and Trump have engaged in a heated rivalry in recent weeks as the brash billionaire has expanded his lead in the GOP nomination fight, and several GOP party elders -- most notably Mitt Romney
-- have implored Republican voters to stop Trump's march to the nomination.
Last week, Rubio joined in on the #NeverTrump hashtag
, which is used by those who say they will not support Trump if he's the GOP nominee. But at the Republican debate on Thursday, Rubio changed course, saying he would support Trump were he to become the Republican standard-bearer.
The next day, Rubio told Kentucky Sports Radio
that his use of the #NeverTrump hashtag meant he was never voting for Trump in the Republican primaries.
Trump was a notable no-show at CPAC this year, where he's spoken in years past. He was scheduled to attend on Saturday, but instead campaigned in Kansas.
A small but vocal minority of attendees at this year's CPAC consider Trump to be a threat to conservatism and the Republican Party. Their ranks include students, lifelong conservative activists, professional Republican political consultants and bloggers, many of whom say they will stay home on Election Day and even some who claim they will help Democratic front-runner Hillary Clinton in a general election campaign over Trump.
Not all on the right are willing to pledge opposition to Trump. His supporters filled seats at CPAC, although not in the same numbers as those for Rubio or Texas Sen. Ted Cruz. And his backers here contend that the strong reactions against Trump are based in fear of his challenges to the status quo.