A growing number of Rubio supporters told CNN privately and publicly that Rubio made a strategic blunder by getting into a personal mud-slinging contest with the bombastic billionaire, arguing that he should not have raised questions about Trump's character -- on everything from criticizing the size of his hands to calling him a "con artist." They argued it only seemed to backfire and make the high-minded and substantive Rubio look petty and unpresidential, fearing it could now be fatal to his presidential aspirations.
"I don't think he needed to get down, deep and dirty," said Utah Sen. Orrin Hatch, a Rubio supporter. "I think he knows that was a mistake. It was against his personality, and I think that hurt him a little bit."
Added Sen. Dean Heller of Nevada, another Rubio backer: "I think the writing is on the wall: aggressively going after a candidate, or a single candidate in this race, probably wasn't as helpful as he wanted it to be."
"He always looked young," said one senior GOP senator. "But that time, he acted young."
Rubio spokesman Alex Conant declined to comment about campaign tactics. For months, Rubio avoided Trump at all costs, worried that bashing the businessman would serve little benefit to him and instead help his rivals. But, according to several advisers and people close to the campaign, Rubio calculated that it made sense to mount a vigorous assault on Trump in late February since the field began to narrow and the Florida senator didn't have to fend off as many attacks from his rivals.
After Ted Cruz, Lindsey Graham, Jeb Bush and others failed to make much of a dent against Trump despite vigorous attacks on the real estate mogul's core convictions, Rubio decided to level a new line of attack: Trump's business record, contending the GOP front-runner simply got rich by hurting average Americans and scamming the system. And to Rubio's team, attacking Trump on a personal level only seemed to energize his core supporters, prompt a flood of money online and inject himself directly into the daily news cycle.
Speaking to CNN's Dana Bash last week at the Conservative Political Action Conference, Rubio defended his sometimes racy attacks.
'Punch them back'
"Donald Trump, he might have grown up the way he did with a lot of money and going to boarding schools," Rubio said. "I can tell you this: Where I grew up, if someone keeps punching someone in the face, eventually someone's going to have to stand up and punch them back."
But on Wednesday, he acknowledged that he's not "entirely proud" of his rhetoric.
"My kids were embarrassed by it," he told MSNBC, adding that "if I had to do it again I wouldn't."
Polls have yet to show a dramatic drop in Rubio's favorability since he sharpened his attacks against Trump. According to a Washington Post-ABC News poll conducted in early March, when Rubio's Trump attacks were fresh and dominating the news cycle, Rubio saw little change to his personal standing, with a 39% favorable rating and 45% unfavorable -- about the same from early January.
But many of his core supporters are now second-guessing his campaign tactics following his poor showing in Tuesday night's primaries -- where he has yet to be awarded a single delegate -- and polls that show him getting trounced in next week's critical Florida primary.
The fear: Attacking Trump has only served to elevate Ohio Gov. John Kasich. And after Rubio's attacks against Cruz's trustworthiness seemed to damage the Texas senator in the South Carolina and Nevada contests, Cruz seems now to be the beneficiary of Rubio's laser-like focus on Trump instead.
Some top Rubio supporters say the GOP senator should hammer home his core message that attracted many to him initially -- his foreign policy expertise and his lofty vows to grow the conservative movement -- and avoid the back-and-forth with Trump.
"Win by what you stand for and how you're going to get the job done," said Sen. Thom Tillis, a North Carolina Republican who still believes Rubio can win the nomination. "If Marco stays focused on that, we've got a great chance."
Staying in through Florida
Despite pressure mainly from his rivals that Rubio should quit before the Florida primary, senior campaign officials tell CNN that there is no chance that he will. They say that the race is remarkably fluid and dispute the findings of a CNN/ORC poll out Wednesday that showed Rubio down 40%-24% to Trump in the Sunshine State. (A Quinnipiac poll released Wednesday showed Trump up by 23 points over Rubio.)
Senior Rubio advisers and people close to the campaign believe the race is much tighter and more in line with a Monmouth poll this week that showed Rubio down by 8 points. Such a lead, according to top Rubio officials, is most certainly surmountable. While Trump may carry absentee voters, Rubio officials have been encouraged that they have been able to get their supporters out to the polls to vote early.
They believe the barrage of anti-Trump attacks flooding the Florida airwaves will damage the front-runner, including the $9 million already spent on advertising by Rubio's super PAC, Conservative Solutions PAC -- far more than any other group in the state. Rubio's team is banking on game-changing moments in Thursday night's CNN debate in Miami, hoping that could help the senator close the gap in Florida. They are likening the Sunshine State to his dramatic comeback in Virginia last week when he lost to Trump by less than 3% of the vote despite polls showing him losing big in the commonwealth.
But unlike Virginia, Florida is a winner-take-all primary, meaning he must defeat Trump to capture all 99 delegates. If he loses, even his supporters concede there's virtually no path to the GOP nomination for Rubio. Some think it's already too late.
"It's over," said one dejected GOP senator who wants Rubio to win.
Arizona Sen. Jeff Flake, a Rubio supporter, said he still is banking on his colleague to pull off a win in Florida. But he was uncomfortable at Rubio's attacks on Trump.
"I probably would have stopped at, 'He wants to make the country orange.' That was pretty good," Flake said, referring to Rubio's joke about Trump's physical appearance. Asked about Rubio's "con artist" attacks on Trump, Flake said, "I don't know about that."
Sen. John Thune, the No. 3 GOP leader who is neutral in the race, said that Rubio is an "attractive" candidate who would help his colleagues in tough Senate races. But he noted that the Trump attacks might have altered the outcome.
"People, for whatever reason, get kind of turned off by it," Thune said when asked about Rubio's anti-Trump tactics. "The level of the rhetoric, the harshness of it -- they start gravitating to other candidates."