Republican presidential candidate John Kasich defended his deal with rival Ted Cruzto divide up three state primary contests, in an attempt to block front-runner Donald Trump from winning the Republican Party's presidential nomination.

On 24 April, the Cruz and Kasich campaigns announced a deal to concentrate their efforts and resources in state contests where each has a better shot. Cruz will focus on Indiana's 3 May primary without competition from Kasich, while Cruz will stand aside in favour of Kasich in Oregon's 17 May primary and New Mexico's 7 June contest.

Kasich told reporters during a campaign stop at a diner in Philadelphia on 25 April: "This is a matter of resources. And you know, we are running a national campaign and we want to apply our resources where we think they can be used most effectively, and it is all designed to stop Hillary Clinton from becoming president."

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 18 July. Trump reacted to the unusual agreement on 25 April, 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."

Asked to react to Trump's attack, Kasich questioned whether Trump knew the meaning of the word "collusion". "I don't respond to Donald Trump. I mean, what are you kidding me?," he said. "Does he know what that means?"

Trump faces a tough path to earn the 1,237 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 8 November presidential election is "rigged" against him, a charge he repeated on 25 April. 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. While some groups opposing Trump welcomed the Cruz-Kasich pact, which some Republicans have urged for weeks, other political strategists questioned whether the deal comes too late.