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A top aide to Senator Marco Rubio on Friday urged his supporters in Ohio to back Gov. John Kasich in that state’s primary on Tuesday, giving fresh momentum to efforts to stop Donald J. Trump a day after a debate in which his rivals declined to take a swing at the leading Republican presidential candidate.

Alex Conant, Mr. Rubio’s spokesman, made the comments in an interview with CNN. He said that he hoped supporters of Mr. Kasich and of Senator Ted Cruz would support Mr. Rubio in his home state primary in Florida, and that he would suggest Mr. Rubio’s backers in Ohio do the same by supporting Mr. Kasich there.

“I’m just stating the obvious,” Mr. Conant said. “If you are a Republican primary voter in Ohio and you want to defeat Donald Trump, your best chance in Ohio is John Kasich, because John Kasich is the sitting governor, he’s very close to Donald Trump in some of the polls there.”

He said the reverse was true in Mr. Rubio’s home state, suggesting that supporters of Mr. Kasich and Mr. Cruz back Mr. Rubio there.

The remarks dovetail with a strategy proposed by Mitt Romney, the party’s 2012 nominee, who urged Republicans opposing Mr. Trump to coalesce around the leading non-Trump candidate in coming nominating contests to deny the nomination to the Manhattan businessman.

Mr. Conant said that “John Kasich is the one candidate in Ohio that can beat Donald Trump — that’s stating the obvious, that is indisputable.”

In a news conference in West Palm Beach, Mr. Rubio echoed the sentiment, but did not go as far in urging his supporters to back Mr. Kasich in Ohio.

“Clearly John Kasich has a better chance than winning Ohio than I do,” Mr. Rubio told reporters. And if Ohioans concluded that the best way to stop Mr. Trump was to vote for Mr. Kasich, the Florida senator said, “I expect that’s the decision they’ll make.”

Asked directly if he was urging his people to vote for Mr. Kasich in Ohio, he said of his spokesman, “I’ll leave it for John to make that argument.”

And Mr. Cruz, speaking in Orlando on Friday, dismissed the strategy, indulging in a laugh when asked about it.

“It’s the Washington establishment’s last gasp: ‘Let’s divide things up. Let’s play games,’” Mr. Cruz said in a taped interview with Fox News. “It’s real, real simple. How do you beat Donald Trump? You beat him.”

Mr. Cruz argued, as he has for weeks, that he was the only candidate still capable of doing so.

Those hoping to defeat Mr. Trump acknowledge that if he wins either Ohio or Florida, it becomes much harder to deny him the nomination.

But Chris Schrimpf, a spokesman for Mr. Kasich, naturally welcomed the idea of having Mr. Rubio’s supporters back the governor in Tuesday’s voting in Ohio.

“We agree with the Rubio campaign that the best chance to beat Donald Trump in Ohio is by voting for John Kasich, and in that spirit, Senator Rubio should immediately tell his Super PAC to stop attacking the governor.”

Matt Flegenheimer and Thomas Kaplan contributed reporting.