Republican front-runner Donald Trump scored an important victory over rival Ted Cruz in Indiana on Tuesday, a win that moves him close to being unstoppable in his march to the party's presidential nomination.

The New York billionaire was quickly projected to be the winner by television networks shortly after polling places closed in the Midwestern state. Trump was on track to take well over 50 percent of the vote, eclipsing Cruz, a US senator from Texas. Ohio Governor John Kasich was running a distant third.

On the Democratic side, Hillary Clinton and Bernie Sanders were running virtually neck and neck, with Clinton's early lead eroded as more votes rolled in.

Cruz had been counting on a win in Tuesday's primary to slow the New York businessman's progress toward the nomination. But Trump rode momentum from wins in five Northeastern states a week ago into a big lead in Indiana over Cruz, whose brand of Christian conservatism had been expected to have wide appeal in the state.

"Lyin' Ted Cruz consistently said that he will, and must, win Indiana. If he doesn't he should drop out of the race-stop wasting time & money," Trump tweeted ahead of a victory speech he was to deliver at Trump Tower in New York.

At Cruz's evening event in Indianapolis, there was an air of resignation among some of his supporters.

"I'm definitely disappointed that we couldn't pull it out for him in Indiana," said Andrew Coulter.

Coulter, 43, said he believes Cruz will continue forward, but gave little hope that his candidate would prevail. "I think that it's at this point Trump is going to win the nomination. I think that this was Cruz's firewall," he said.

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

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

Campaigning in Evansville, in the state's southwest corner, Cruz sounded deeply frustrated by the bombastic real estate mogul, who has ripped Cruz at every turn.

"The man cannot tell the truth but he combines it with being a narcissist," Cruz said, "a narcissist at a level I don't think this country has ever seen."

Cruz termed Trump a "serial philanderer" - likely as part of his strategy to try to win the support of evangelical voters. Trump, in response, said Cruz had become "more and more unhinged."

The only hopes that Cruz and Kasich have for becoming the Republican nominee is to somehow deny Trump the 1,237 delegates he needs to win the nomination outright and force Republicans at their July convention in Cleveland to choose one of them.

Kasich vowed to stay in the race.

"Tonight's results are not going to alter Gov. Kasich's campaign plans," Kasich senior strategist John Weaver said in a campaign memo. "Our strategy has been and continues to be one that involves winning the nomination at an open convention." – Reuters