The move was the latest sign that Trump continues to occupy the center of the GOP race and affect virtually every major strategic decision in the contest. Heading into next Tuesday's crucial slate of primaries, Trump is well-positioned to add to his delegate lead and could deal a knockout blow to Rubio and Kasich by beating them in their home states.

But there were no immediate signs of a smooth coordinated effort between the campaigns and outside groups aligned against Trump. Kasich's campaign did not respond to Rubio's comments warmly. Meanwhile, a pro-Rubio super PAC did not bow to the Kasich campaign's subsequent demand that it stop running television ads against the Ohio governor.

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The string of events highlighted how difficult it will be to build the kind of large-scale movement against Trump that Mitt Romney and some other Republicans are urgently calling for.

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

Ohio is among Tuesday's biggest prizes, awarding its 66 delegates on a winner-take-all basis. Polls show a close race between Kasich and Trump, with Rubio and Sen. Ted Cruz (R-Tex.) well behind. If Trump loses it would mark the most fruitful victory of the campaign by anti-Trump forces.

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On Tuesday, Florida will award all 99 of its delegates to the winner. Polls show Trump leading Rubio.

The Florida senator said he had not spoken with Kasich or made any arrangements with him. His proposal is similar to what Romney, the 2012 GOP nominee, 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.

But Kasich's campaign did not seem inclined to go along, illustrating the difficulty involved in coordinating a anti-Trump movement across multiple states and involving more than one campaign.

"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, Sen. Rubio should immediately tell his Super PAC to stop attacking the governor," said spokesman Chris Schrimpf.

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Asked for a response, the pro-Rubio super PAC Conservative Solutions, which has bought TV air time in Illinois and Florida for anti-Kasich ads, did not say it would ease up.

"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.

For Rubio, there was a self-serving incentive to launch the unconventional strategy. He is desperately trying to win as many anti-Trump voters as he can in his home state. Had Kasich agreed to return the favor in Florida by asking his backers to vote for Rubio, it could have potentially given him a boost here. Even Rubio's most loyal backers privately concede that he is effectively finished if he loses. .

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