Cruz, meanwhile, said Kasich was "pulling out" of the state. A super PAC supporting the Texas senator also said it would continue to air an anti-Kasich ad in the state - a sign the Cruz camp fears Kasich could still get enough support to sink Cruz's chances here.
The tumult fuelled doubts about the arrangement among voters and Republican elites, who are concerned Cruz and Kasich have handed Trump a ready-made argument that the party establishment is plotting against him.
The mogul said as much in a series of stump speeches on the eve of primary voting today, when he is poised to rack up delegates in five Eastern states: Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island.
"Honestly, it shows such total weakness, and it's pathetic when two longtime insider politicians - establishment guys, whether you like it or not - have to collude, have to get together to try to beat a guy that really speaks what the people want," Trump said.
Cruz and Kasich are bound by two self-serving goals: winning enough delegates between them to prevent Trump from clinching the nomination outright and trying to demonstrate that he isn't favoured by most Republicans.
If those goals are met, July's convention in Cleveland is likely to be a free-for-all offering Cruz and, perhaps, Kasich a shot at the nomination.
Many Republicans were dubious.
"Voters tend to stick to their preferred candidate for personal, not tactical, reasons," said former Minnesota Governor Tim Pawlenty.
The deal was immediately put to the test yesterday when Kasich said any Indiana voter inclined to vote for him should do so. "I've never told 'em not to vote for me," he said.
Just hours earlier, his campaign co-chair in Indiana, Jim Brainard, was saying the opposite. "Kasich is asking his supporters in Indiana to vote for Cruz so Trump does not win Indiana."
The initial announcements did not detail what the candidates' voters should do, but the intent was clear: to try to help Cruz win Indiana on May 4 and to help Kasich claim Oregon on May 18 and New Mexico on June 8.
Cruz said the division of resources "made sense from both campaigns."
Up for grabs today
Pennsylvania
The biggest prize, Pennsylvania will award 17 delegates to whichever Republican wins the state - almost certainly Donald Trump. The 54 delegates left will be unbound at the convention in July, but the better a candidate does in the primary the most hand-picked delegates he can send to the convention. For the Democrats, it's worth a whopping 189 delegates to be awarded proportionately. Hillary Clinton is expected to take the lion's share.
Maryland
The statewide winner on the Republican side will win 14 delegates, with the remaining 24 being allocated to the winners in each of the state's Congressional districts. Trump could win anywhere from 26 to all 38 delegates. The Democrats will award 118 delegates in Maryland, with Clinton expected to take most.
Connecticut
Like in neighbouring New York and Massachusetts, Trump is expected to dominate in Connecticut. The big question is whether he can hit 50 per cent and so take all or nearly all of the state's 28 delegates. If not, they'll be allocated proportionately and Trump will miss a chance to narrow the gap to 1237. Polls show Bernie Sanders within striking distance of Clinton. A win here would help him make the case that the race is still on.
Delaware
Though a small state, Delaware awards its delegates in a winner-take-all format. Trump is all but guaranteed to take all 16. Another state where Clinton is expected to win, but Sanders is close enough to hold out hope for an upset.
Rhode Island
The outcome is proportional, with Trump is expected to take just over half of the 19 delegates on offer. This is perhaps Sanders' best chance to pull out a win, with the most recent poll showing him in the lead. If he doesn't fare well elsewhere it may be too little too late.
- Telegraph Group Ltd, Washington Post-Bloomberg
