Donald Trump has blasted rivals Ted Cruz and John Kasich for reaching a deal to divide up three state primary contests in an attempt to block the frontrunner from winning the Republican Party's presidential nomination.
The real estate mogul and reality television personality reacted to the unusual agreement by saying it was "sad" that the two fellow Republicans had to team up to in order to try to defeat him.
"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," Trump said in a statement.
"They are mathematically dead and this act only shows, as puppets of donors and special interests, how truly weak they and their campaigns are."
The Cruz and Kasich campaigns announced a deal yesterday to concentrate their efforts and resources in state contests where each has a better shot.
Cruz will focus on Indiana's May 3 primary without competition from Kasich, while Cruz will stand aside in favour of Kasich in Oregon's May 17 primary and New Mexico's June 7 contest.
Cruz, a US senator from Texas, and Kasich, Ohio's governor, hope their efforts will weaken Trump in those states and keep him from securing enough delegates to claim the Republican nomination before the party convention beginning July 18.
The deal comes as a handful of mid-Atlantic states prepare for primary elections on Tuesday.
Trump faces a tough path to earn the 1237 delegates needed to lock up the nomination before the convention.
While candidates can win a state contest, they often must still win over delegates who often are allocated at separate events.
Republicans will pick their delegates in at least four states this weekend, including Alaska, Arkansas, Arizona and Virginia.
Trump has frequently complained that the process for choosing a nominee for the Nov. 8 presidential election is "rigged" against him, a charge he repeated on Monday. Party officials have said the rules have long been known.
If no candidate has enough support on the first vote at the national convention, many delegates can switch sides on subsequent ballots, opening up a potential free-for-all.