Republican presidential candidates Ted Cruz and John Kasich sent out near-simultaneous statements announcing a collaborative effort to block rival and front-runner Donald Trump in Indiana, Oregon and New Mexico's primaries. (Jenny Starrs/The Washington Post)

BORDEN, Ind. -- Donald Trump's two remaining Republican presidential opponents campaigned Monday after striking an agreement on a strategy to divvy up three states holding primaries in the coming weeks -- an unusual and urgent arrangement aimed at stopping the mogul from clinching the GOP nomination.

The campaigns of Sen. Ted Cruz of Texas and Ohio Gov. John Kasich released written statements within minutes of each other Sunday night calling for Kasich to stop competing in Indiana and for Cruz to clear the way for Kasich in New Mexico and Oregon. They called on allied third-party groups to do the same.

Speaking to reporters here in Borden before a Monday morning rally, Cruz said it was "big news today that John Kasich has decided to pull out of Indiana to give us a head-to-head contest with Donald Trump." He said the division of resources in key primary states "made sense from both campaigns."

Republican presidential candidate Ted Cruz defended his alliance with rival John Kasich April 25. Cruz said their attempt to block front-runner Donald Trump from winning the party's nomination "makes sense." (Reuters)

Kasich, meanwhile, was careful to describe the deal as a matter of shifting resources -- and not as a signal to his voters in Indiana that they should back Cruz.

"I’ve never told ‘em not to vote for me," Kasich told reporters at a breakfast stop at Philadelphia's Penrose Diner. "They ought to vote for me. But I'm not over there campaigning and spending resources."

The deal marked the most significant collaboration between anti-Trump forces yet. It came as Trump stood on the verge of another week of sweeping victories, with five Eastern states where he is favored to win set to vote Tuesday. It sparked swift condemnation from the front-runner, who accused his rivals of "collusion in order to stay alive."

In recent weeks, Cruz had vehemently rejected the idea of cutting a deal to outflank Trump, and has openly criticized Kasich's continued presence in the race despite winning only his home state. But after Trump's commanding win in New York last week, there is growing anxiety among his critics that a win by the mogul in Indiana could devastate their efforts to stop him. Kasich and Cruz are trying to trigger a contested Republican convention in Cleveland where party activists would decide the GOP nominee.

The deal was discussed in a private meeting last week between Cruz campaign manager Jeff Roe and Kasich chief strategist John Weaver in Hollywood, Fla., on the sidelines of the Republican National Committee meeting, said a source with knowledge of the discussions who spoke on the condition of anonymity. The deal was finalized Sunday in phone calls between the two advisers.

"To ensure that we nominate a Republican who can unify the Republican Party and win in November, our campaign will focus its time and resources in Indiana and in turn clear the path for Gov. Kasich to compete in Oregon and New Mexico, and we would hope that allies of both campaigns would follow our lead," Roe said in a statement.

A few moments later, Weaver issued a statement saying: "We are very comfortable with our delegate position in Indiana already, and given the current dynamics of the primary there, we will shift our campaign’s resources West and give the Cruz campaign a clear path in Indiana."

He added: "In turn, we will focus our time and resources in New Mexico and Oregon, both areas that are structurally similar to the Northeast politically, where Gov. Kasich is performing well. We would expect independent third-party groups to do the same and honor the commitments made by the Cruz and Kasich campaigns."

Trump issued a statement early Monday morning saying: "Collusion is often illegal in many other industries and yet these two Washington insiders have had to revert to collusion in order to stay alive. They are mathematically dead and this act only shows, as puppets of donors and special interests, how truly weak they and their campaigns are."

Meanwhile, anti-Trump groups said they were heartened by the arrangement.

"Encouraging," Tim Miller, a spokesman for the anti-Trump Our Principles PAC, said of the Cruz-Kasich deal. "See you in Cleveland."

In a statement late Sunday, Rory Cooper, senior adviser of the group #NeverTrump, said: "Whether you support Ted Cruz or John Kasich, a second ballot at the Convention is imperative to stopping Donald Trump. We're happy to see the Kasich and Cruz campaigns strategically using their resources to deny Donald Trump delegates where they are in the strongest position to do so."

A version of the idea already was effectively in place. Cruz skipped campaigning altogether in Rhode Island, Connecticut and Delaware last week -- three states that will vote on Tuesday. Kasich campaigned in the former two states.

Cruz took sharp aim at Trump on Monday in his question-and-answer session with reporters and his rally, saying that if Trump is the nominee, Republicans would risk losing control of the U.S. House and Senate and jeopardize the Bill of Rights.

The move was expected to send Kasich west sooner than his rivals expected. Campaign sources said that two planned Tuesday events in the Indianapolis area would be canceled; the candidate would attend a local fundraiser, then move on.

Cruz had already shifted his focus beyond Tuesday's contests to Indiana, which will vote on May 3. He held several campaign events in the Hoosier State last week and plans more this week, beginning with four stops on Monday.

Limited public polling in Indiana has shown that Cruz stands the best chance of stopping Trump. Cruz allies believe Kasich is siphoning votes away from him, as evidenced by a pro-Cruz TV ad released last week that urged voters not to back Kasich.

A loss to Trump in Indiana would be a catastrophic blow to Cruz, who is hoping to undercut some of Trump's momentum heading into the final month of the race.

Oregon will vote on May 17. New Mexico's primary is June 7.

Both the Cruz and Kasich campaigns said they intend to compete hard in the other remaining states, signaling that the scope of their unorthodox arrangement will be limited. The biggest remaining prize is California on June 7. The state will award 172 delegates.

Trump currently leads the Republican race with 845 delegates, according to the latest Associated Press count. Cruz has 559, and Kasich is much further back, with 148. To win the nomination outright, a candidate must clinch 1,237 delegates.

If no one reaches that mark, a contested convention would be triggered and in subsequent balloting, delegates bound to certain candidates would be freed up to to vote for someone else. Cruz has excelled at identifying and elevating individuals to serve as delegates who would be loyal to him under such a scenario.

Trump has complained about the delegate process, arguing it is "rigged."

This post has been updated

Weigel reported from Washington. Philip Rucker contributed to this report