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."

Watch more!

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."

LEFT: SOCTIA, NY - APRIL 7: Republican presidential candidate Ted Cruz loosens his tie which speaking to supporters after a townhall at Mekeel Christian Academy in Scotia, New York on Thursday, April 7, 2016. Cruz begins campaigning in New York state after his decisive win in Wisconsin on Tuesday. (Photo by Cassi Alexandra/For The Washington Post) ------ RIGHT: POUGHKEEPSIE, NY - APRIL 17: Republican presidential candidates Donald Trump speaks during a campaign event at the Mid-Hudson Civic Center in Poughkeepsie, NY on Sunday April 17, 2016. (Photo by Jabin Botsford/The Washington Post) (Left: Cassi Alexandra/For The Washington Post, RIGHT: Jabin Botsford/The Washington Post) WASHINGTON, DC - JULY 28: Republican presidential candidate, U.S. Sen. Ted Cruz (R-TX) speaks during a Anti-abortion rally opposing federal funding for Planned Parenthood in front of the U.S. Capitol July 28, 2015 in Washington, DC. Planned Parenthood faces mounting criticism amid the release of videos by a pro-life group and demands to vote in the Senate to stop funding. (Olivier Douliery/Getty Images) NEW YORK, NY - APRIL 19: Republican presidential candidates Donald Trump waves after speaking at a campaign press conference moments after winning the republican presidential primary at Trump Tower in New York, NY on Tuesday April 19, 2016. (Jabin Botsford/The Washington Post) Sen. Ted Cruz, R-Texas talks with Susan B. Anthony List President Marjorie Dannenfelser after he spoke at the Susan B. Anthony List "Campaign for Life Gala and Summit", a gathering of anti-abortion advocates, in Washington, Wednesday, March 12, 2014. (AP Photo/Charles Dharapak) SYRACUSE, NY - APRIL 16: Republican presidential candidates Donald Trump speaks during a campaign event at the Nicholas J. Pirro Convention Center in Syracuse, NY on Saturday April 16, 2016. (Jabin Botsford/The Washington Post) SOCTIA, NY - APRIL 7: Republican presidential candidate Ted Cruz speaks during a townhall at Mekeel Christian Academy in Scotia, New York on Thursday, April 7, 2016. Cruz begins campaigning in New York state after his decisive win in Wisconsin on Tuesday. (Cassi Alexandra/For The Washington Post) U.S. Senator Ted Cruz (R-TX) (2nd L) and his wife Heidi Cruz greet businessman Donald Trump (L) onstage as they address a Tea Party rally against the Iran nuclear deal at the U.S. Capitol in Washington September 9, 2015. Both Cruz and Trump are U.S. Republican presidential candidates. REUTERS/Jonathan Ernst - RTSD94 (� Jonathan Ernst / Reuters/REUTERS) PHILADELPHIA, PA - APRIL 19, 2016: Republican Presidential candidate Ted Cruz held a Pennsylvania Kickoff Event in Philadelphia, Pennsylvania on Tuesday, April 19, 2016. He spoke with the media following the event. (Michael Robinson Chavez/The Washington Post) HARTFORD, CT - APRIL 15: Republican presidential candidates Donald Trump speaks during a campaign event at the Connecticut Convention Center in Hartford, CT on Friday April 15, 2016. (Jabin Botsford/The Washington Post) Photo Gallery: Although Trump has voiced myriad eyebrow-raising positions, Ted Cruz has at times taken more surprising stances.

This post has been updated