PHOTO: Huffington Post
The sudden alliance, revealed in short statements, arose due to the pressing timing of the Republican party's presidential primary season.
Trump, a wealthy property developer and reality television star, has pushed close to amassing the number of delegates to the party convention that would ensure he will be the Republican standard-bearer.
Trump, campaign reject charges he is fooling voters
Cruz's campaign manager Jeff Roe said the campaign would "focus its time and resources in Indiana and in turn clear the path for Governor Kasich to compete in Oregon and New Mexico." Kasich's team put out a similar statement.
"Both know their best shot at preventing Trump from clinching the nomination outright is to team up to block his path and force a contested convention. And it may still be too late," Politico reported.
A dismissive Trump said on Twitter: "Wow, just announced that Lyin' Ted and Kasich are going to collude in order to keep me from getting the Republican nomination. DESPERATION!"
Some influential party figures such as 2012 nominee Mitt Romney have aligned with a Stop Trump movement, which may or may not be benefiting Trump's chief rival Cruz, an arch-conservative US senator from Texas.
Trump, Clinton aim to seal deal in Tuesday primaries
Cruz told reporters Trump has been "lying to us" and is pretending to be a conservative in order to "fool gullible voters."
Barely 36 hours before voters in five states head to the polls, Trump lashed out at Cruz, accusing him of "bribing" all-important delegates as part of the convoluted primary system for choosing the Democratic and Republican nominees.
Trump has repeatedly described the process as rigged, and has mocked the party for allowing campaigns to bestow gifts such as flights and dinners on delegates.
Hillary Clinton is increasingly seen as the presumptive Democratic Party choice.
Earlier, Trump and his presidential campaign pushed back Sunday against accusations by rivals in both parties that the celebrity billionaire is a political fraud who has been misleading American voters.
The Republican frontrunner's new senior advisor Paul Manafort raised eyebrows when he told Republican heavyweights at a closed-door meeting that Trump has been playing a "part" in front of rally audiences and that the role was "evolving" into a more serious and policy-focused one.
Likening it to the "Wizard of Oz" children's tale, Democratic National Committee chair Debbie Wasserman Shultz on Sunday mocked it as "basically Toto pulling the curtain on the wizard, and revealing that Donald Trump isn't on the level, has never been on the level."
"They basically have a faker running for president of the United States," she told "Fox News Sunday."
Manafort went on the same channel to try to quell the furor over his remarks, which leaked Thursday.
Donald Trump uses Indian accent to mock call centre representative
He insisted that Americans were seeing "the real Donald Trump in campaign mode talking to people," and that the New York real estate mogul was not out to mislead anyone.
"We were evolving the campaign, not the candidate, and the settings were going to start changing," he told Fox.
Trump's incendiary campaign -- he has called some Mexicans "rapists", vowed to build a wall on the southern US border and wants to bar Muslims from entering the country -- has infuriated the Republican establishment.
Trump said Cruz was busy focusing on the behind-the-scenes wooing of delegates in the event there is no outright winner heading in to July's Republican convention in Cleveland.
"They had boats and yachts waiting to take delegates around," Trump said at a rowdy rally in Hagerstown, Maryland, speaking of the Republican National Committee's spring meeting this past week with delegates gathered at a Florida resort.
"We want to put it (nomination) away," Trump said. "I only care about the first ballot. We're not going for the second and third and fourth and fifth."
Trump is leading in Republican polls in all five states that vote Tuesday: Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island.
Trump leads substantially in the delegate battle, with 846 delegates compared to 563 for Cruz and 147 for third-place John Kasich.
A candidate must secure 1,237 delegates to win the Republican nomination outright.
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