Donald Trump has hit out at a collaboration between Ted Cruz and John Kasich aimed at unifying the anti-Trump vote in some remaining primaries.
Mr Trump said his rivals are colluding in a way that would be illegal in many industries.
Under the arrangement, Mr Kasich, the Ohio governor, will step back in the Indiana contest next week to let Mr Cruz bid for voters who do not back Mr Trump.
Mr Cruz, a Texas senator, will do the same for Mr Kasich in Oregon and New Mexico.
Mr Trump said in a statement that Mr Cruz and Mr Kasich are “mathematically dead” – meaning neither can gather enough delegates to clinch the nomination before the party’s July convention – and their pact smacks of desperation by two “puppets of donors and special interests”.
The arrangement does not address the five north-eastern states set to vote on Tuesday, where Mr Trump is expected to add to his already overwhelming delegate lead.
However, the shift offers Trump foes a glimmer of hope in their long and frustrating fight to halt the billionaire’s rise.
“Having Donald Trump at the top of the ticket in November would be a sure disaster for Republicans,” Mr Cruz’s campaign manager, Jeff Roe, said in a statement explaining the new plans.
“Not only would Trump get blown out by (Hillary) Clinton or (Bernie) Sanders, but having him as our nominee would set the party back a generation.”
Mr Kasich’s chief strategist, John Weaver, added: “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 announcement marks a sharp reversal for Mr Cruz’s team, which aggressively opposed co-ordinating anti-Trump efforts with Mr 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 Mr Kasich backs out of Indiana, Mr Cruz promised he would not compete in Oregon on 17 May and New Mexico on 7 June.
Mr Trump campaigned in Maryland on Sunday, which will vote tomorrow along with four other states, Rhode Island, Connecticut, Pennsylvania and Delaware.
Speaking to several thousand people in an aeroplane hangar in Hagerstown, Maryland, Mr Trump stressed repeatedly 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.”