It also disagreed with Trump being a 'popular' candidate, arguing that Democratic presidential front-runner Hillary Clinton has received more votes than him and many Republicans have not voted for the real estate tycoon.

"We would call on everyone to refrain from violence. And there's no political justification, no political dispute among citizens that would justify an act of violence. There's no denying that," White House Press Secretary Josh Earnest told reporters at his daily news conference.

I noted that it was even the Wall Street Journal editorial board who observed that Secretary Clinton has got a million more votes than Trump has," he said.

"So I recognise that's a fact that he does not enjoy seeing discussed particularly often, but it's true. It is why the President will be a forceful advocate in support of a Democratic nominee who emerges, because the stakes of the next election will be significant," he said.

Earnest said the President will have a forceful case to make that the values and priorities that have been championed by Democrats on the campaign trail are critically important to the future success of the country, "particularly in the comparison to the rhetoric and values that are be espoused by a variety of Republican candidates, not just one."

Trump on Wednesday warned of riots if he is denied the Republican presidential nomination, after adding to his growing pile of primary wins but losing in the key state of Ohio.

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