WEST PALM BEACH, Fla. — Florida Sen. Marco Rubio on Friday gave the green light to his supporters in Ohio to vote for their Gov. John Kasich in Tuesday's Republican presidential primary in order to slow the momentum of GOP front-runner Donald Trump, an unusual tactic that comes as Rubio is trying to rally the anti-Trump vote in Florida behind his candidacy.

Earlier in the day, Trump gained a significant ally: retired neurosurgeon Ben Carson, who endorsed his former rival, describing him as a “very intelligent man that cares deeply about America.”

While Rubio stopped short of actively urging his Ohio backers to vote for Kasich, he did not try to convince those who think their governor represents the best chance to defeat Trump there not to cast their ballots for him.

“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.

The Florida senator said he had not spoken with Kasich or made any arrangements with him.

His proposal is similar to what 2012 Republican presidential nominee Mitt Romney called for in a recent anti-Trump speech. Romney said voters who want to prevent Trump from claiming the nomination should vote for Rubio and Kasich in their respective states.

Rubio said that voting for Kasich or Texas Sen. Ted Cruz in Florida, which also holds its GOP primary Tuesday, amounts to a vote for Trump.

“The only one that has a chance to beat Donald Trump in Florida is me,” Rubio said.

He accused Trump of not having “the basic knowledge, not to mention the competency or the temperament” to be commander in chief.

Polls show Rubio trailing Trump in Florida and Kasich in a close race with the real estate mogul in Ohio.

Kasich spokesman Rob Nichols charged that his candidate is going to win in Ohio without Rubio's help “just as he's going to lose Florida without our help.”

During a Friday morning news conference at Trump's Palm Beach resort, Mar-a-Lago, the GOP front-runner introduced Carson, who said that Americans need to know that there are “two Donald Trumps.”

One is the man who has dominated headlines and the Republican race with his brash statements, said Carson, who captivated some conservative voters with his unconventional presidential campaign before dropping out of the race last week.

The other is “very cerebral and sits there and considers things very carefully.”

“He's much more cerebral and a much more reasonable person than comes across,” Carson said of Trump. As voters “begin to see the real individual there,” he said, “I think we're going to be comforted as a nation.”

Echoing a plea that Trump has made himself in recent days, Carson implored GOP leaders to coalesce around the controversial front-runner.

“A house divided against itself cannot stand,” Carson said, echoing the biblical reference once cited by Abraham Lincoln, in allusion to some Republican leaders who have said they will not back Trump.

Carson and Trump blamed their earlier friction during the 2016 race on the nature of politics, with Carson saying he was not personally offended.

“We move on because it's not about me, it's not about Mr. Trump,” he said.

For his part, Trump praised Carson as “a special, special person” who has led “an awe-inspiring life.”

Trump said that he and Carson had a good discussion on education this week and that he hopes to keep Carson involved in his campaign and potential presidency.

As a member of the Seventh-day Adventist Church who frequently drops biblical references, Carson's endorsement could help boost Trump's support among evangelical Christian voters while hurting Trump rival Cruz, who counts evangelicals as part of his base.

Carson has had a rocky relationship with Cruz after accusing the senator of tricks to dissuade voters from backing the retired neurosurgeon on the night of the Iowa caucuses.

This week, Cruz won the endorsement of another candidate who dropped out of the race, former Hewlett-Packard Chief Executive Carly Fiorina.

Another onetime candidate, former Florida Gov. Jeb Bush, is said to be meeting with Trump's rivals, although it is unclear whether Bush will endorse anyone ahead of Florida's GOP primary Tuesday.

Meanwhile in Ohio, a judge Friday granted a request to let 17-year-olds who will be 18 before the fall election vote Tuesday.

Democratic presidential candidate Bernie Sanders' campaign — which had filed a separate federal lawsuit over the limitation — praised the judge's ruling.

Tribune Newspapers' Kate Linthicum in Miami and Associated Press contributed.