But now Rubio says he's willing to drive around the country in a "pickup truck" to stop the "sham artist" named Trump from winning the GOP nomination.
"The party of Lincoln and Reagan will never be held hostage by a con artist," the senator from Florida told supporters in Miami on Tuesday night.
With a do-or-die moment facing his campaign in mid-March, Rubio has gone from indifference to urgency in dealing with the GOP's front-runner, a shift in strategy that underscores how mainstream Republicans have failed to find the right tactics to derail the Trump campaign.
In particular, his two closest rivals, Rubio and Sen. Ted Cruz of Texas, spent much of the last year avoiding attacking Trump, a move some believe allowed the bombastic billionaire to define himself early and gain a head of steam that some fear now makes him unstoppable heading to the GOP nomination.
Some of Rubio's closest allies privately fear the Florida Republican waited too long -- and that his newfound focus on Trump is the only way to revive a campaign that is on the ropes.
"Should they have gone after him? Yeah. Would it have been effective? I don't know," said one of Rubio's Senate colleagues. "We don't know this is going to be effective. Maybe there isn't a magic tea to tell voters that Donald Trump is a huckster."
Senior Rubio campaign officials and people close to the campaign said Rubio had little choice but to wait until after the field narrowed to go after Trump. They said they were taking sustained fire from the likes of Jeb Bush, Chris Christie and Ted Cruz, and were forced to respond in kind.
Moreover, they argued, had Rubio focused on Trump with a more crowded field, it only would have served to benefit Trump, who was dominating the media coverage, and would have done little to improve Rubio's own standing.
Rubio sources point to Bush, Sen. Lindsey Graham, Sen. Rand Paul, then-Louisiana Gov. Bobby Jindal and former Texas Gov. Rick Perry -- all of whom launched the most vigorous assaults on Trump early in the campaign season yet none of whom are still in the race. With a bigger field, attacking Trump didn't seem to benefit the candidate lobbing the biggest grenades, senior Rubio officials concluded.
"You can't fight everyone at the bar at the same time, you got to clear the others out first," said one GOP strategist.
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Rubio alluded to as much when talking to reporters in between campaign events on Tuesday, while acknowledging he was "caught off guard" by the rise of Trump.
"Well, I had hoped in this process we would have -- the voters would wake up and see what Donald Trump is, a con artist," Rubio said. "And I think we were all caught off guard by how much the attention in coverage has really lifted him."
Rubio added: "So you've had this artificial buildup of him, and I think finally the time had come" to launch new attacks. Rubio added that until last week he was taking "incoming attacks from multiple candidates" but now it's time to "unmask" who Trump really is.
To do that, Rubio's team and outside allies plan to focus extensively over the next two weeks on Trump's failed business ventures and how they have affected ordinary Americans.
Rubio allies believe Trump's use of bankruptcy laws and the failure of Trump University, among other issues, have not been properly litigated in the public sphere. And outside groups attacking Trump are planning a sustained assault on the airwaves on these matters in the runup to the winner-take-all primary contests on March 15, particularly in Rubio's home state of Florida, according to people familiar with the matter.
Yet some Republicans believe Rubio's decision to dive into the mud pit, particularly by hurling personal insults, will do nothing to help the Florida freshman.
"I've always thought that [Trump is] exposed on policy -- not the size of his hands, but the ill-conceived nature of his policies," said Sen. Lindsey Graham, R-South Carolina, referring to Rubio challenging Trump's manhood over the last several days.
Still, massive amounts of air time are being reserved to attack Trump, particularly in Florida, sources said. And because Rubio began his Trump assault during CNN's debate in Houston last week, Rubio's campaign and super PAC, Conservative Solutions PAC, each have been raising a massive amount of cash each day, sources say. Supporters are more energized, according to several people close to the campaign.
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"I think it's an effective line of attack," Sen. Cory Gardner, R-Colorado and a leading Rubio surrogate, said of focusing on Trump's business dealings. "That's why you're starting to see more people show at rallies, that's why you're starting to see networks covering him live. They weren't before."
Gardner added: "Absolutely, he can be caught. There's no doubt about that. What I worry about is we're going to people in this country and making sure there is no inevitable Donald Trump."
Despite Trump dominating on Super Tuesday and marching toward the nomination, the Rubio campaign believes Trump is getting weaker. They credit the growing attacks for weakening Trump in states like Oklahoma, which Cruz won, and Virginia, where Rubio finished a strong second. And they believe Trump will only grow weaker after two debates and a paid advertising assault.
But they acknowledge Florida is turning into a must-win state; winning there will help deny Trump the necessary delegates to win the nomination, effectively pushing the fight into the GOP convention in July.
Still, many believe it's a long shot. And some Republicans worry the field is too big and that it still must narrow further before Trump can be derailed.
"It's pretty clear that multiple candidates are fragmenting the vote, in a way that probably helps Mr. Trump," said Texas Sen. John Cornyn, the No. 2 Republican in the Senate.