"I've used strong words," he said. "My parents didn't raise me that way, but I feel passionate about it."
Rubio, whose inspirational story and speech-making have made him a presidential contender, has gambled that he can hammer Trump without changing his image. His Rodney Dangerfield-esque treatment of Trump has punched through Trump's news cycle and -- more importantly, as far as Rubio was concerned -- goaded the front-runner into responding with a long, applause-free monologue about how it was unfair to call him a con man. But Rubio's voters, who skew more educated than Trump voters, are not cheering for the election to turn into an insult contest.
"He's kind of bringing the Trump game to Trump, and no other candidate has done that successfully," said Edwin Chaput, 25, who previously supported Sen. Rand Paul (R-Ky.) and is now unsure of his vote.
"He's now having to play the game that Trump plays, but I hate that Trump forced his hand," said John Wright, 28. "One thing I like about him is that insults do not come naturally to him."
"I wish that being above reproach, like Rubio is, could win it for you," said Laura Powell, 24, Wright's fiance.
In Birmingham, Rubio kept his anti-Trump case to policy, and to the "con" of a wealthy mogul exploiting people with Trump University's pricey real estate class, and exploiting undocumented immigrants to build his flagship skyscraper. "He's exploited the system forever, and now he's the champion of it?" Rubio asked.
As Rubio took serious questions, the suntanned specter of Trump seemed to vanish. Given the chance to clean up his remarks on "drafting" women into the military, Rubio said it was largely a distraction.
"Never again will there be a war where we have to draft hundreds of thousands of people," Rubio said. I don't even think we need Selective Service anymore."
When a student government leader at Samford University told Rubio that she had benefited from Deferred Action from Childhood Arrivals, he didn't budge on his promise to undo it, but speculated that Congress could quickly pass legislation to sort out situations like hers.
"The most compassionate people in the world will look at a story like this and say: We can figure this out," he said.
Only once more did Trump come up -- as a joke. After Rubio detailed his policies on student loans, YellowHammer News founder Cliff Sims could not quite resist a follow-up.
"Would Trump University qualify?" Sims asked.
"No, they would not," Rubio said crisply. "And their football team was 0-10 every year."