But hoping to tap into the voter unrest that has propelled his rivals, the Florida senator more recently began displaying an angrier, gloomier side. He has painted a grim portrait of a "great nation in decline," particularly when it comes to terrorist threats endangering national security.
The results have been mixed: Negative Rubio has not amounted to a more popular Rubio.
He has failed so far to sway voters away from billionaire
As a result, Rubio's attempt to balance his once-cheerful message with a more ominous one risks diminishing the very quality that gave rise to his star power -- without reaping the kind of rewards Trump and Cruz have found by putting an exclamation point on voter fears.
"This is hard for me say this, but it's the truth and you need to hear it: Your country is a nation in decline," Rubio said in mid-January at New England College, according to New Hampshire Public Radio. "It is headed in the wrong direction, and it's headed in the wrong direction fast."
A day earlier, at the Republican debate in South Carolina, Rubio's new angrier tone was apparent. He warned, "Let me tell you, if we don't get this election right, there may be no turning back for America."
Rubio only occasionally flashed his telegenic smile during a national security speech Monday in Des Moines, where he promised that when he is president, "We're going to have a real war on terror."
Matt Strawn, a former GOP chairman in the state, is worried Rubio's new tone may backfire. He was glad to see the candidate pivot during the speech to close with the message of hope embodied in his parents' immigrant story.
"That's what we need to hear more of," said Strawn. "I don't know if he needs to paint a picture of a dark and scary world. It's on our TV every day."
Rubio's presidential bid started with so much optimism it drew comparisons to Barack Obama, another young senator who had a gift of oratory.
Many supporters say Rubio's forward-looking tone – "yesterday's gone" -- was part of what attracted them to his candidacy in the first place. They see him as the kind of messenger the GOP needs to regain the White House.
But after the terror attacks in Paris and San Bernardino, and with Trump and Cruz dominating the polls, Rubio allies say he needed to harden his message to avoid sounding out of touch with the mood of the GOP electorate.
"It resonates with people -- I know it did with me," said Ted Weaver, a city councilman in the Des Moines suburb of Clive, who believes Rubio's tougher tone helps compensate for his "youth and his boyish looks," which may be a "hindrance" to some voters.
Weaver said voters have "very much responded to his message, his language, his tone of concern."
The new assertiveness also came after Rubio was widely ridiculed for wearing some high-ish heeled black boots, which other candidates quickly mocked as not very manly.
Rivals have seized on the new Rubio to suggest the campaign is adrift, scrambling to reach new audiences in the final days before voting begins.
"People like Rubio who smiles, and who's enthusiastic, who's youthful and energetic, " said Cruz advisor Rick Tyler. "People haven't seen that Rubio in a while."
The question now is which Rubio will show up at Thursday's final debate before voting begins in Iowa.
Opponents also note that while Rubio talks tough on national security issues, he skipped many of the Senate Foreign Relations Committee meetings that he has boasted give him the international expertise needed to be commander-in-chief.
A pro-Jeb Bush group, Right to Rise, has pounded Rubio with negative ads over his Senate voting record, hoping to capitalize on Iowans' strong work ethic.
Rubio's spokesman said the senator's attendance is in line with other senators, and that Foreign Relations Committee meetings are often held at the same time as the mostly classified Intelligence Committee briefings, which Rubio viewed as a higher priority.
The senator's message of optimism has remained the same, said spokesman Alex Conant. "His idea that America's a great country, that hasn't changed," said Conant. "He always leaves the audience believing the future can be better than the past."
With national security rising as a top issue among voters, Rubio was smart to begin portraying himself as a strong candidate who can "keep us safe," said Bobby Kaufmann, a state representative in Iowa who is backing Rubio.
"I don't think he's lost all his optimism… but he has to focus," Kaufmann said. "It would be tone deaf to stick to your original campaign message and be unable to reflect world events."
Supporters believe Rubio will finish strong in the Hawkeye State. But even when he won the coveted endorsement of the influential Des Moines Register, the newspaper's editorial noted its preference for the candidate's original approach.
"At his best, Rubio offers an uplifting message of a 'new American century,'" the newspaper wrote. "Yet more recently, he has pandered to rising pessimism in his party…. We hope Marco Rubio and his party take a different path, one that can lead to the opportunity and optimism he so eloquently articulates."