Invigorated from a feisty debate performance that put Trump on the defensive, Rubio pulled in money and endorsements as he sought to reinforce his argument that he is the Republican establishment's best hope for uniting the fractured party and stopping the billionaire's march to the nomination.
Trump fired back in characteristic fashion, stealing the limelight for at least a few hours with the surprise endorsement of New Jersey Gov.
Reveling in the rare support from an establishment Republican, Trump let loose on Rubio, calling him a "nervous Nellie" who is unfit for the presidency.
The rivals mocked each other's heavy use of television makeup; Trump also ridiculed Rubio for his profuse sweating at times.
"Can you imagine [Russian President
The back-and-forth outbursts of personal invective signaled the enormous stakes as the nomination race hurtles into its most critical phase, the two-week period that starts Tuesday when the biggest swath of states hold primaries and award the most delegates to date.
Trump leads polls in the dozen states that vote March 1, on Super Tuesday, with the exception of Texas, home to Sen.
Name-calling aside, Rubio faces a fundamental problem: He has yet to win a single state. Polls show him trailing Trump even in Florida, his home state.
"To win, you have to win. And you don't win by doing well," said Stuart Stevens, the top strategist to the GOP's 2012 nominee, Mitt Romney.
Polls show a growing number of Republicans believe Trump will be the nominee, while Rubio remains most voters' second choice.
"We're past the point of trying to lower expectations," said Kevin Madden, a GOP consultant and Romney's former spokesman. "When you've got someone with Donald Trump's momentum, you've got to start beating expectations."
Rubio embraced that strategy for the first time Thursday night when he unexpectedly unleashed a fierce and unrelenting barrage against Trump at a debate in Houston. Trump leveled his own charges, but Rubio's jabs clearly stung.
Over and over, he assailed Trump's hiring of undocumented foreign workers to build Trump Tower in New York, the state's fraud investigation and lawsuits aimed at the now-defunct Trump University, his four corporate bankruptcies, the constant audits of his tax returns, the millions of dollars he inherited from his father, and other concerns dug up by campaign researchers.
"There is no way we can allow a con artist to take over the conservative movement, and Donald Trump is a con artist," Rubio said, repeating that charge in a fundraising letter and a round of TV show interviews Friday morning.
The billionaire dismissed Rubio as "childlike" and a "lightweight" in a series of tweets Friday morning.
Trump's brash language and unconventional style have forced Rubio to abandon, at least in part, a campaign plan based on a sunny, Reagan-like message of economic opportunity. In recent weeks, he has hardened his position on immigration and joined Trump in calling Cruz a liar.
While polls show many voters are late deciders, others are fixed in their views as the once-vast Republican field narrows. It's unclear whether Rubio's shift in tactics will be enough to turn the tide.
Many in the GOP say Rubio needs to win at least one or two of the March 1 contests — made up of mostly Southern states that have largely embraced Trump's anti-establishment appeal — or risk falling hopelessly behind in delegates needed to win the nomination.
Rubio was once considered a sure bet to win the Republican caucuses in Nevada, where he spent part of his childhood and campaigned heavily. But Trump scored a sizable victory there Tuesday after Rubio failed to win Iowa, New Hampshire and South Carolina.
Rubio has tried to spin his four defeats into success stories, insisting they prove he is the party's top alternative to Trump. Cruz, who bested Trump in Iowa, makes the same argument for his campaign.
Backers say they always expected Rubio to trail early and then gradually build momentum. But Trump's early winning streak has forced Rubio's campaign to rethink its electoral math.
Early on, Rubio's team privately argued that he only needed to do well enough in Iowa and New Hampshire, the first two contests, to push on to victories in friendlier states, South Carolina and Nevada. They called it the 3-2-1 strategy, for his expected finishes in the first three states.
But Rubio fell to fifth in New Hampshire after Christie needled him so mercilessly in a debate that he appeared to be in distress. A series of high-profile endorsements, and a public mea culpa for his performance, helped keep his campaign alive but couldn't help him overtake Trump in South Carolina.
Rubio's aides insist they are not worried.
"The key to us was to clear the field and get in a one-on-one campaign with Trump, and that's what's happening," said Rubio spokesman Alex Conant, although Cruz, Ohio Gov.
Rubio has won endorsements from several U.S. senators as well as Arkansas Gov. Asa Hutchinson and former Minnesota Gov.
Former Florida Gov. Jeb Bush's exit from the race has allowed high-dollar donors to move to Rubio's side. Bush is expected to endorse Rubio before the state's primary on March 15, a must-win state for Rubio.
But by then, it may be too late.
Times staff writer Michael Finnegan contributed to this report from Los Angeles.
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