Donald Trump says an extraordinary collaboration between Ted Cruz and John Kasich aimed at unifying the anti-Trump vote in some remaining primaries is a desperate move by "mathematically dead" rivals.

Such collusion would be illegal in many industries, the Republican presidential front-runner said, but it's illustrative of "everything that is wrong in Washington and our political system."

Under the arrangement outlined late Sunday, Kasich, the Ohio governor, will step back in the May 3 Indiana contest to let Cruz bid for voters who don't like Trump. Cruz, a Texas senator, will do the same for Kasich in Oregon and New Mexico.

The arrangement does not address the five Northeastern states set to vote Tuesday, where Trump is expected to add to his already overwhelming delegate lead. Yet the shift offers increasingly desperate Trump foes a glimmer of hope in their long and frustrating fight to halt the billionaire's rise.

"It is big news today that John Kasich has decided to pull out of Indiana to give us a head to head contest with Donald Trump," Cruz told reporters as he campaigned in Indiana on Monday. "That is good for the men and women of Indiana. It's good for the country to have a clear and direct choice."

Kasich sent mixed messages, however, as he addressed the pact for the first time while campaigning in Philadelphia.

Asked what Indiana voters should do next week, the Ohio governor just 13 hours after the arrangement was announced urged them to vote for him.

"I've never told them not to vote for me. They ought to vote for me," Kasich said in a Philadelphia diner. He said he simply agreed not to spend "resources" in Indiana.

The announcement marks a sharp reversal for Cruz's team, which aggressively opposed coordinating anti-Trump efforts with Kasich as recently as late last week. And the agreement applies only to Indiana, Oregon and New Mexico - three of the 15 states remaining on the Republican primary calendar. As Kasich backs out of Indiana, Cruz promised he would not compete in Oregon on May 17 and New Mexico on June 7.

Cruz's campaign manager, Jeff Roe, said in a statement explaining the new plans that Trump would be soundly defeated by the Democratic nominee, whether it's Hillary Clinton or Bernie Sanders. "Having Donald Trump at the top of the ticket in November would be a sure disaster for Republicans," he said.

Added Kasich's chief strategist, John Weaver, "Our goal is to have an open convention in Cleveland, where we are confident a candidate capable of uniting the party and winning in November will emerge as the nominee."

Trump was set to campaign in Rhode Island and Pennsylvania on Monday, two of the five states hosting primary contests on Tuesday. Connecticut, Delaware and Maryland are also voting.

Speaking to several thousand people Sunday evening in Maryland, Trump stressed repeatedly that he expects to win the 1,237 delegates needed in the first round of voting in Cleveland to stave off a contested convention.

"I only care about the first," he said. "We're not going for the second and third and fourth and fifth."

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Peoples reported from Washington. Associated Press writers Kathleen Ronayne in Manchester, New Hampshire, and Errin Haines Whack in Philadelphia contributed to this report.