FILE - Republican presidential candidates, from left, Texas Senator Ted Cruz and Ohio Governor John Kasich participate in a Republican presidential debate at the University of Miami, in Coral Gables, Fla., March 10, 2016.
FILE - Republican presidential candidates, from left, Texas Senator Ted Cruz and Ohio Governor John Kasich participate in a Republican presidential debate at the University of Miami, in Coral Gables, Fla., March 10, 2016.

The effort to prevent billionaire Donald Trump from becoming the Republican Party's nominee for U.S. president took a new step late Sunday as his two chief rivals openly detailed their plan to work against the front-runner in three states.

Since winning the February 9 New Hampshire primary, Trump has led the Republican race as candidates try to take over the White House when President Barack Obama's term ends early next year.

But Trump's ascendant campaign, written off early as something that would quickly fade, has made many in the party uncomfortable both because of his controversial statements on immigration and foreign policy and questions about how wedded he is to the party and its positions.

With less than three months before the Republican convention in Cleveland, Trump sits as the only candidate with a mathematical chance of reaching the majority 1,237 delegates needed to clinch the nomination.

Republican presidential candidate Donald Trump spe
FILE - Republican presidential candidate Donald Trump speaks during a campaign rally in Bridgeport, Conn., April 23, 2016.

That has not stopped Texas Senator Ted Cruz or Ohio Governor John Kasich from continuing their campaigns.

They have repeatedly cast Trump as dangerous for the party and said that if he is chosen, then the Democratic candidate – likely former Secretary of State Hillary Clinton – would easily win the November general election.

On Sunday they announced their plan to keep Trump from hitting the 1,237 mark. 

Divide and conquer

Within a span of 15 minutes, the Kasich and Cruz campaigns released statements saying Kasich will step aside and leave the May 3 primary in the Midwestern state of Indiana to Cruz.

Indiana has increasingly appeared as the key in knowing whether the bid to stop Trump will succeed. It has 57 delegates, 30 of which will go to the overall winner in the state. The rest will be divided up based on the winner in each congressional district.

A collection of polls from last week put Trump ahead in Indiana with about 39 percent support, compared to 33 percent for Cruz and 19 percent for Kasich. If the Cruz-Kasich plan goes as planned, Cruz could cruise to a win and deny Trump an important batch of delegates.

The Cruz campaign said the agreement with Kasich is about ensuring the Republican nominee is someone who can unite the party.

Republican presidential candidate Sen. Ted Cruz of
FILE - Republican presidential candidate Sen. Ted Cruz of Texas speaks at a campaign rally at Gateway High School in Monroeville, Pa., April 23, 2016.

"Having Donald Trump at the top of the ticket in November would be a sure disaster for Republicans. Not only would Trump get blown out by Clinton or [Vermont Senator Bernie] Sanders, but having him as our nominee would set the party back a generation," Cruz campaign manager Jeff Roe said.

Trump has repeatedly criticized the Republican Party as being unfair to him throughout the campaign. He called the effort by Cruz and Kasich "sad" and said they have to use collusion to keep their presidential bids going.

"Because of me, everyone now sees that the Republican primary system is totally rigged. When two candidates who have no path to victory get together to stop a candidate who is expanding the party by millions of voters, (all of whom will drop out if I am not in the race), it is yet another example of everything that is wrong in Washington and in our political system,” Trump said.

In return for not competing hard in Indiana, Kasich is getting a pledge that Cruz will similarly step aside and not campaign in the Western states of Oregon and New Mexico.

Oregon has 28 delegates at stake May 17, while New Mexico and its 24 delegates are up for grabs on June 7. 

Game plan 

"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," the Kasich campaign said. "We believe that will be John Kasich, who is the only candidate who can defeat Secretary Clinton and preserve our GOP majority in the Congress."

A boy wearing a Ted Cruz cap shakes hands with U.S
FILE - A boy wearing a Ted Cruz cap shakes hands with U.S. Republican presidential candidate John Kasich during a campaign event in Syracuse, New York, April 18, 2016.

Both the Cruz and Kasich campaigns said they will compete hard in every other remaining state.

If Trump fails to reach a majority in the first round of voting at the convention, then delegates are free to cast their vote differently on subsequent ballots, meaning even Kasich, who trails far behind in delegates, could have a chance at the nomination.

Polling on potential general election matchups suggests that if the November vote were to happen today, Republicans would be better off with either Cruz or Kasich.

Major polls have Clinton beating Trump by about 10 points and Cruz by about two points, but losing to Kasich by eight points.

Sanders, who has his own tough battle in trying to erase Clinton's big delegate lead, beats all three Republicans in the polls, with the margin closest against Kasich.

Delegate counts for U.S. presidential candidates,