Declaring the Republican presidential contest at “a fork in the road,” Ted Cruz and John Kasich have defended their extraordinary new alliance as the party’s last, best chance to stop Donald Trump, even as the New York billionaire surged toward another big delegate haul.

Mr Trump, the Republican front-runner, lashed out at what he called collusion by desperate rivals, intensifying his attacks on the GOP presidential nomination system on the eve of Tuesday’s round of primary elections in the Northeast.

“If you collude in business, or if you collude in the stock market, they put you in jail,” Mr Trump said as he campaigned in Rhode Island.

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“But in politics, because it’s a rigged system, because it’s a corrupt enterprise, in politics you’re allowed to collude.”

“It shows how pathetic they are,” he said of his Republican rivals.

Connecticut, Pennsylvania, Delaware and Maryland hold primaries on Tuesday along with Rhode Island.

Mt Cruz, a Texas senator, and Mr Kasich, the Ohio governor, announced the terms of an unprecedented agreement late on Sunday night (local time) to co-ordinate primary strategies in three of the 15 remaining primary states.

Donald Trump (left) and Ted Cruz have escalated their bitter feud. Photo: Getty

Mr Kasich will step back in the May 3 Indiana contest to let Mr Cruz bid without interference for voters who don’t like Mr Trump.

Mr Cruz will do the same for Mr Kasich in subsequent contests in Oregon and New Mexico.

The arrangement does not address Tuesday’s primaries, where Mr Trump is expected to add to his already hefty delegate lead.

Yet the shift offers increasingly desperate Mr Trump foes a glimmer of hope in their long and frustrating fight to keep him from amassing enough delegates to seal his nomination and avoid a contested national convention in July.

Ignoring the Northeast on Monday, Mr Cruz insisted, “We are at a fundamental fork in the road,” as he campaigned in Indiana.

“It is big news today that John Kasich has decided to pull out of Indiana to give us a head-to-head contest with Donald Trump,” the fiery conservative told reporters.

“That is good for the men and women of Indiana. It’s good for the country to have a clear and direct choice.”

The plan carries risks – especially as Mr Trump bashes a “rigged” nomination system.

Mr Trump is the only Republican candidate who can clinch the GOP presidential nomination before his party’s national convention.

Yet his path is narrow.

The front-runner needs to win at least four of the five Northeastern states on Tuesday.

He enters the day with 845 delegates, 392 short of the 1237 needed to represent his party in the general election in November.

Eliminated from reaching that total in the primaries, Mr Cruz and Mr Kasich can only hope to block Mr Trump from reaching a majority – and a first-round convention victory – and thus force a contested convention where delegates could select a different nominee.