Republican presidential candidate Marco Rubio's campaign is urging supporters to vote against Donald Trump in Ohio, and instead back John Kasich. (Reuters)

The increasingly precarious movement to slow Donald Trump’s steady march toward the Republican presidential nomination received a new and unexpected boost Friday when Sen. Marco Rubio (R-Fla.) freed his Ohio supporters to vote for Ohio Gov. John Kasich, who stands the best chance of notching a win over Trump in Tuesday’s critical contest.

But there were no signs of additional major efforts by the campaigns or outside groups aligned against Trump, highlighting the difficulty of waging a large-scale push against the mogul. Kasich’s campaign didn’t return the favor to Rubio in Florida, for example, and a pro-Rubio super PAC would not agree to cease its attacks on the governor there.

Meanwhile, Trump added a new and prominent name to his growing roster of high-profile backers: retired neurosurgeon Ben Carson, who endorsed Trump at his posh Mar-a-Lago resort five miles from where Rubio launched his desperation move.

Touting his national security credentials at a synagogue, Rubio stopped just short of explicitly urging Ohio voters to back Kasich — but his meaning was clear.

“Clearly, John Kasich has a better chance of winning Ohio than I do, and I think if a voter in Ohio concludes that voting for John Kasich gives us the best chance to stop Donald Trump there, I anticipate that’s what they will do,” Rubio said.

John Kasich and Marco Rubio desperately need to win their home states on March 15th – or their paths to the GOP nomination will be a lot tougher. (Peter Stevenson/The Washington Post)

Ohio is the second-biggest winner-take-all prize Tuesday, offering 66 delegates to the victor. Polls show a close race there between Kasich and Trump.

Former presidential nominee Mitt Romney, a staunch Trump critic, has urged voters to cast ballots strategically to maximize Trump opposition as the nominating contest winds through the remaining states. He has called for Ohio voters to rally around Kasich and Florida voters to support Rubio.

But if Rubio was hoping that Kasich would greenlight his Florida backers to support the senator, he didn’t get his wish.

After a rally with 1,000 supporters at a glass factory outside Dayton, Ohio, Kasich dismissed a question about whether he would ask his backers to vote for Rubio in Florida.

“If I’ve got supporters somewhere in the country, and I’m on the ballot, they kind of ought to vote for me,” he said. “I mean, what kind of deal would it be if I told my people, ‘Don’t vote for me’?”

Earlier, Kasich spokesman Chris Schrimpf urged a pro-Rubio super PAC, Conservative Solutions, to stop running ads against Kasich in Florida and Illinois. The super PAC rejected the idea.

“We’re focused on ensuring Floridians are clear about what’s at stake on Tuesday — a vote for Ted Cruz or John Kasich in Florida is a vote for Donald Trump, plain and simple,” said Jeff Sadosky, a spokesman for Conservative Solutions.

Speaking at a rally in Hialeah, Fla., Republican presidential candidate Marco Rubio vowed to "be on that ballot on Tuesday." Florida is among five states holding presidential primary contests on March 15. (Reuters)

Cruz, the senator from Texas who is second in the delegate count behind Trump, could be the last Trump opponent standing if both Kasich and Rubio lose their home states. He campaigned in Orlando with Carly Fiorina on Friday.

Here in Florida — Tuesday’s biggest prize, with all 99 delegates going to the winner — Rubio trails Trump in every recent public poll. Even Rubio’s supporters admit that a loss would effectively end his campaign.

Rubio’s campaign, which was already struggling to take off, took a nose dive recently after he launched a wave of nasty personal attacks against Trump, including mocking his “small hands” and implying that another body part might be small. He later said he regretted his tactics.

On Friday, Rubio kept his assault on Trump focused on policy, as he did in Thursday night’s debate. He accused the mogul with lacking “the basic knowledge, not to mention the competency or the temperament,” to be commander in chief.

“I think he just wants to finish this as his authentic self,” GOP strategist Ana Navarro, a friend of Rubio’s who said she was in touch with the candidate this week, wrote in an email. “Marco is not a schoolyard bully. He’s an optimistic, policy-focused, good guy. Him admitting he’d made a mistake going in the gutter to attack Trump, and that it embarrassed his kids, was a very human moment.”

At Mar-a-Lago in Palm Beach, Trump appeared with Carson for a 45-minute news conference. Trump had harshly criticized Carson when the retired neurosurgeon was in the race, accusing him of having a “pathological disease” with no cure, similar to being a “child molester.”

But Carson said that he and the businessman had “buried the hatchet” and that there are “two different” Trumps: the one the public sees and a more “cerebral” Trump behind the scenes.

Trump praised Carson and said he would play a “big, big part” in his campaign, in both political and policy capacities. He declined to offer specifics.

The real estate billionaire also called for the GOP to unify around his candidacy and said he sees no need for any more debates. “I think we’ve had enough debates. How many times do you have to give the same answer to the same questions?” Trump said.

Carson, who has a winter home in nearby West Palm Beach, visited Trump at Mar-a-Lago on Thursday morning for breakfast. Over a spread of pastries, fruit and coffee — and without aides present — the two men spoke for more than an hour.

Many Rubio supporters fear it is too late for the young Cuban American senator to salvage his run. In Thursday’s debate, Rubio sounded like a candidate who sought to restore his dignity more than anything else.

His opening statement was all about the generational contrast he underscored at the start of his campaign last April and his vision of a “new American century.” Later, he eagerly embraced a policy debate with Trump on Cuba, where his parents were born, and he closed with a nod to the dreams they had for him.

After watching his three rivals opine about their paths to the nomination, delegate math and the prospect of a contested convention, he didn’t even bother explaining how he could win. Instead, Rubio relayed a story his wife, Jeanette, told him about a man who recently had surgery and who sits outside early-voting sites with a Rubio sign — despite doctors’ instructions to rest.

Despite the long odds he faces here in Florida, an ebullient Rubio mingled with top donors and supporters such as Sen. Cory Gardner (R-Colo.) Friday morning at the Versailles Restaurant in Miami, where he celebrated his well-received debate performance.

Campaign officials told the group that early voting in Miami-Dade County is breaking 2 to 1 for Rubio and that the campaign had made more than 200,000 calls since Monday.

The atmosphere was similarly upbeat Thursday as about 70 of his financial backers, including hedge-fund billionaire Paul Singer, gathered for a lunch at the Miami Beach home of real estate developer Jimmy Resnick.

Rubio’s daunting challenge is getting voters to tune in as he tries to right the wrongs that sent supporters scattering. His rally in Hialeah, a heavily Cuban American city near Miami, on Wednesday was small, covering only a tiny fraction of the football field where it was held. Rubio will campaign in central Florida and Pensacola over the weekend.

Nelson Diaz, a former Rubio aide and the chairman of the Miami-Dade County Republican Party, praised Rubio’s debate performance.

“More importantly,” he added in a text message, “I think he was very effective at restoring his positive message campaign.”

Matea Gold, Robert Costa and Katie Zezima in Miami; Philip Rucker in Coral Gables, Fla.; and Jim Tankersley in Dayton, Ohio, contributed to this report.