Donald Trump says he’s the inevitable Republican presidential nominee and has warned if party leaders deny him the nomination, “You’d have riots”.

The brash billionaire, who won at least three more states on Wednesday but doesn’t yet have the needed majority of delegates, predicted on Thursday he’d collect enough support before the Republican convention this summer.

Many party leaders are concerned by his comments against Muslims, immigrants and women and the violence at some of his rallies.

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Democrat Hillary Clinton, who moved closer to becoming the first woman in US history to win a major party nomination after winning at least four states Tuesday, took direct aim at Trump.

“Our commander-in-chief has to be able to defend our country, not embarrass it,” Clinton said in a speech that largely ignored rival Bernie Sanders.

The editors of The Washington Post also condemned Trump’s warning in an editorial that implored the Republican Party to thwart his rise to power.

“A democrat disavows violence, a demagogue wields it as a threat. The Republican Party should recognize the difference, and act on it before it is too late,” the Post’s editorial board wrote on Thursday (AEDT).

But the billionaire seems confident that the party establishment will unite behind him. Without naming names, Trump told US morning television that some Republican senators who are publicly critical of him have called privately to say they want to “become involved” in his campaign.

Trump added Clinton doesn’t have the “strength or stamina” to be president, while a new ad posted on his official Instagram account includes footage of Clinton barking like a dog.

The footage from earlier this year was of her barking to mimic an ad she’d been describing to supporters.

The Republican front-runner said he’d skip a debate scheduled for Monday, saying, “I think we’ve had enough debates.” The debate was cancelled after Ohio Governor John Kasich said he also would not attend.

Trump won Tuesday in Florida, North Carolina, and Illinois but fell in Ohio to Kasich.

Clinton triumphed in the Florida, Illinois, Ohio and North Carolina primaries.

Votes were also being counted in Missouri, though races in both parties there were too close to call.

Florida senator Marco Rubio dropped out Tuesday after losing his home state to Trump.

With more than half the delegates awarded through six weeks of primary voting, Trump, who has 621 delegates, is the only Republican candidate with a realistic path to the 1,237 delegates needed to clinch the nomination through the traditional route.

On the Democratic side, Clinton’s victories were blows to Sanders and bolstered her argument that she’s the best Democrat to take on the eventual Republican nominee.

Clinton has at least 1,561 delegates of the 2,383 needed, including the superdelegates who are elected officials and party leaders free to support the candidate of their choice.