"The fact that I'm here now in front of those cameras comes with responsibility, and I'll confess, I've learned that myself," the Florida senator said. "A few weeks ago, in response to some attacks about the size of my ears and something else, I responded in kind with an attack about -- I'm not going to repeat it -- stuff."
"I felt terrible about it," Rubio said. "I realized that win or lose, there are people out there that see what I'm doing and follow it as a role model."
After days of unflinching optimism on the campaign trail, the gravity of Tuesday's Florida primary appeared to bear down at one of Rubio's last stops of the day. And potentially, one of the last stops of his presidential campaign.
If Rubio can't defy the latest polls
and rise to victory here, it could help pave the way for Trump to clinch the nomination and anoint a brand of politics than many Republicans privately say they have come to loathe and even fear.
"Leadership is not about going to angry and frustrated people and saying, 'You should be even angrier and more frustrated, and you should be angry and frustrated at each other,'" Rubio said. "That is not leadership. You know what that is? That's called demagoguery, and it is dangerous."
He lamented Trump's over-the-top style
, from his use of profanity -- "we have never had a presidential candidate that has to be bleeped out" -- to his apparent willingness to pay the legal fees for a supporter who sucker punched a protester.
"I know there are people that like this stuff because he says what they want to be able to say," Rubio said. "Presidents can't say whatever they want to say. You have to be honest, you have to be correct and you have to be truthful. But you can't say whatever you want to say."
The crowd cheered with approval.
Rubio's rally at Palm Beach Atlantic University, a Christian school in South Florida, drew the kind of crowd establishment Republicans have hoped to build into a general election coalition. It was young. It was diverse. Afterwards, college students, suburban women and immigrants all crushed in around the candidate shouting, "Marco! Marco!" and vied for autographs and pictures.
Rubio opened his speech by recalling that 11 months ago this week, he announced his presidential bid at Miami's Freedom Tower, using the iconic site to highlight his immigrant roots and the significance of the American Dream.
Monday night, he made an overt plea for voters to support him -- and an unspoken one, imploring them to stop Trump
As Luis Jimenez emerged from the scrum, he said he was still holding out hope that Rubio could notch a win in Florida. Both he and his wife had already cast early ballots for him.
"He's used to being an underdog," Jimenez, a 44-year-old Boynton Beach resident, said. "There's a long shot."
As he gamed out Rubio's odds, Rossaida Jimenez, 43 years old, shook her head in disagreement.
"He's more positive than I am," she said.
If Rubio doesn't succeed on Tuesday
and Trump wins the GOP nomination, both said they'll cast their ballots for the billionaire in November.
Later Monday, Rubio's bus pulled up to a crowd of hundreds gathered on a brightly lit, outdoor basketball court in West Miami, where Rubio would hold his potentially last rally as a presidential candidate. He hopped up on the bed of a pick-up truck and used a bullhorn to address the enthusiastic crowd, as a giant American flag hung on a building behind him.
It was a homecoming of sorts, given that Rubio said he used to play basketball in this park and it's an area where he campaigned to run for city commissioner two decades ago.
"And this is the park I wanted to be in tonight, on the eve of the most important election in a generation, in the state that always makes the difference, in the part of the state that's going to carry us over the top," he said, talking about Tuesday's primary.
Speaking more in Spanish than in English, Rubio thanked the community for their support over the years. He had fun joking around with the crowd -- especially in Spanish -- and appeared more relaxed and at ease than he has in days.
"No matter where I'll go or where I'll be, I will always be a son of this community," he told them. "I will always carry with me the hopes and dreams of generations who made possible the hopes of mine."