Ted Cruz has suspended his White House campaign after suffering a crushing defeat in Indiana’s primary, leaving the road wide open for Donald Trump to seize the Republican nomination.

“From the beginning, I’ve said that I would continue on as long as there was a viable path to victory,” the Texas senator told supporters in Indianapolis on Wednesday morning (AEST).

“Tonight, I’m sorry to say, it appears that path has been foreclosed.

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“We gave it everything we’ve got, but the voters chose another path.”

The New York billionaire was quickly projected to be the winner by television networks shortly after polling places closed in the Midwestern state.

Mr Trump was on track to take well over 50 per cent of the vote, eclipsing Mr Cruz, while Ohio Governor John Kasich was running a distant third.

Meanwhile, on the Democratic side, Hillary Clinton and Bernie Sanders were running virtually neck and neck.

Mr Cruz had been counting on a win in Indiana to slow the New York businessman’s progress toward the nomination.

But Mr Trump rode momentum from wins in five north-eastern states a week ago into a big lead in Indiana over Mr Cruz, whose brand of Christian conservatism had been expected to have wide appeal in the state.

Trump links Cruz’s father with JFK assassination

The loss for Mr Cruz was a sour ending to a rough day in which he got entangled in a harsh back-and-forth with Mr Trump.

It began when the billionaire repeated a claim published by the tabloid newspaper the National Enquirer that linked Senator Cruz’s father, Cuban émigré Rafael Cruz, with President John F Kennedy’s assassin Lee Harvey Oswald.

Campaigning in Evansville, Indiana, Mr Cruz sounded deeply frustrated by the bombastic real estate mogul, who has ripped the conservative Mr Cruz at every turn.

“This is just kooky. The man is utterly a moron,” Mr Cruz said.

“A caricature of a braggadocious, arrogant buffoon who builds giant casinos with giant pictures of him everywhere he looks.

“The man cannot tell the truth but he combines it with being a narcissist — a narcissist at a level I don’t think this country has ever seen.”

Mr Cruz also termed Mr Trump a “serial philanderer”.

Mr Trump, in response, said Mr Cruz had become “more and more unhinged”.

Thank you Indiana, we were just projected to be the winner. We have won in every category. You are very special people-I will never forget!

— Donald J. Trump (@realDonaldTrump) May 3, 2016