Donald Trump's Republican rivals have announced plans to co-ordinate primary strategies in upcoming states to deprive the front-runner of the delegates he needs to win the party nomination.
Ted Cruz and John Kasich took the extraordinary step of issuing near-simultaneous statements outlining an agreement that may be unprecedented in modern American politics.
The Kasich campaign will give Cruz "a clear path in Indiana". In return, the Cruz campaign will "clear the path" for Kasich in Oregon and New Mexico.
"Having Donald Trump at the top of the ticket in November would be a sure disaster for Republicans," Cruz's campaign manager Jeff Roe said in a statement explaining the new plans.
"Not only would Trump get blown out by Clinton or Sanders, but having him as our nominee would set the party back a generation."
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 arrangement marks a sharp reversal for Cruz's team, which aggressively opposed the idea of a co-ordinated anti-Trump effort as recently as late last week. Yet it underscores a bleak reality for the billionaire businessman's Republican foes - time is running out to stop him.
The announcement came less than 48 hours before voting begins across five north-eastern states where New Yorker Trump is poised to add to his already overwhelming delegate lead.
He campaigned on Sunday in Maryland, which will vote on Tuesday along with Rhode Island, Connecticut, Pennsylvania and Delaware.
Speaking to several thousand people in an aircraft hangar in Hagerstown, Maryland, Trump stressed repeatedly that he expected to win the 1,237 delegates needed in the first round of voting to stave off a contested convention.
"I only care about the first. We're not going for the second and third and fourth and fifth," he said.
Even before the plan was announced, Cruz all but abandoned the north-eastern states in favour of Indiana, which holds its primary on May 3.
Both Cruz and Kasich had cast the state as a critical turning point. Under the terms of the new agreement, however, Kasich will allow Cruz to take on Trump without interference.
As recently as three days ago Kasich's campaign announced investments in Indiana, including the opening of two offices and the creation of a campaign leadership team. And he had planned to campaign there on Tuesday, with a town hall gathering in Indianapolis to watch the results of the primaries. Those events have now been cancelled.
As Kasich backs out of Indiana, Cruz promised to not compete in primary contests in Oregon on May 17 and New Mexico on June 7.
"We will focus our time and resources in New Mexico and Oregon, both areas that are structurally similar to the north east politically, where Governor Kasich is performing well," Weaver said.
Like Cruz's campaign, Kasich's has encouraged allied super PACs - political action committees - and other outside groups to "honour the commitments".
Trump did not immediately respond to the agreement. He has repeatedly condemned the Republican presidential nominating system as "rigged".