The senator has hurled unusually personal attacks at Trump in recent days regarding the billionaire's motivations, his demeanour and even his age. Rubio has been particularly intent on undermining Trump's business record, characterising him as an entitled heir whose success was based on his family's connections and money.
"I very rarely try to attack other Republicans because I don't like it; I think the Democrats love when we do that. I think they cheer when we do that," Rubio said, defending his recent shift in strategy. "But we've come to a point now where we have no choice but to talk about these differences."
Senator Ted Cruz, speaking in Atlanta, told supporters Wednesday would be the "most important day in this entire cycle". At stake, he said, was whether the Republican Party would be able to fend off Democratic front-runner Hillary Clinton. "If we nominate Donald," he said, "we end up electing Hillary as President."
Cruz, at a rally in Little Rock, cited a lawsuit that accused Trump of knowingly hiring undocumented immigrants to work on Trump Tower. Trump lost the lawsuit, appealed and it was ultimately settled.
"He put his name on it," Cruz said of Trump Tower. "Maybe it should say underneath: 'Built by illegal immigrants'."
But Trump's conservative credentials have been bolstered by a series of high-profile endorsements. New Jersey Governor Chris Christie joined Trump in Bentonville, Arkansas, yesterday, one day after throwing his support behind the billionaire. The Trump campaign announced that former Arizona Governor Jan Brewer, a Republican firebrand who rose to national prominence because of her hardline stance on immigration reform, had also endorsed him. Those endorsements could be the first of many by such Republican leaders.
Trump fought back against those attacks in Bentonville, where he and Christie took turns criticising Rubio.
"I watched this lightweight Rubio, total lightweight, little mouth on him, 'bing, bing, bing'," Trump said, parodying Rubio's attacks. "The last thing I am is a con man."
Trump faced sustained attacks backed by millions of dollars in South Carolina but emerged unscathed with a double-digit victory. But as the target on his back grows, a national advertising effort across multiple states could wear away at his populist support.
Trump's critics acknowledge that the 11th-hour attacks from Rubio and others may come too late to make a difference this week.
But that is not to say they are in vain. Trump's detractors hope that sustained attacks against his business record and character will stymie his momentum ahead of the March 16 primary contests, many of which award delegates on a winner-take-all basis.
Katie Packer, a former deputy campaign manager for Mitt Romney who leads an anti-Trump super PAC, said she believed Rubio had a clear path forward.
"We have no idea if [Trump is] impenetrable because nobody has ever tried to take him down," she said.
