WASHINGTON - The feud between Donald Trump and Ted Cruz hit high note Tuesday as Republican primary voters headed to the polls in Indiana.

Trump, the Republican presidential front-runner,  was asked on Fox News to respond to remarks by the senator's father, Rafael Cruz,  who said electing “the alternative” to his son “could be the destruction of America.”

“I think it’s a disgrace that he’s allowed to do it, I think it’s a disgrace that he’s allowed to say it,” Trump responded.

He alleged that Cruz’s father was with John F. Kennedy’s assassin shortly before he murdered the president, visiting a National Enquirer story claiming that Rafael Cruz was pictured with Lee Harvey Oswald handing out pro-Fidel Castro pamphlets in New Orleans in 1963.

A Cruz campaign spokesperson told the Miami Herald, which pointed out numerous flaws in the Enquirer story, that it was “another garbage story in a tabloid full of garbage.”

“His father was with Lee Harvey Oswald prior to Oswald's being - you know, shot. I mean, the whole thing is ridiculous,” Trump said Tuesday during a phone interview with Fox News. “What is this, right prior to his being shot, and nobody even brings it up. They don't even talk about that. That was reported, and nobody talks about it.”

“I mean, what was he doing - what was he doing with Lee Harvey Oswald shortly before the death? Before the shooting?” Trump continued. “It’s horrible.” Trump’s tangent followed his rebuke of Rafael Cruz using the pulpit to court evangelicals for his son.

“I implore, I exhort every member of the body of Christ to vote according to the word of God and vote for the candidate that stands on the word of God and on the Constitution of the United States of America,” Rafael Cruz said in a video clip aired by Fox News. “And I am convinced that man is my son, Ted Cruz. The alternative could be the destruction of America.”

 “You look at so many of the ministers that are backing me, and they’re backing me more so than they’re backing Cruz, and I’m winning the evangelical vote,” Trump said. “It's disgraceful that his father can go out and do that. And just - and so many people are angry about it. And the evangelicals are angry about it, the way he does that.”

“But I think it's horrible,” he added. “I think it's absolutely horrible that a man can go and do that, what he's saying there.” 

Reuters adds: Meanwhile, New York billionaire Donald Trump expects Republican primary voters who cast ballots on Tuesday in Indiana to make him unstoppable in his march toward the party's presidential nomination.

"If we win Indiana, it's over," Trump told a cheering crowd in Terre Haute, Indiana, on the eve of the vote.

The blunt-spoken real estate mogul holds a double-digit polling lead over US Senator Ted Cruz of Texas, who has been campaigning in the Midwestern state almost nonstop since mid-April. Cruz has trumpeted Indiana, one of the last big states left in the fight to get onto the Nov. 8 presidential ballot, as his golden moment to stop Trump and force a brokered nomination at the party's July convention. But it appears to be shaping up as his Waterloo.

Fresh off a sweep of five Northeastern states last week, Trump wants a win in Indiana to put him within reach of the 1,237 delegates required to lock up the Republican nomination before the convention.

Trump now has 996 delegates, compared with 565 for Cruz and 153 for Ohio Governor John Kasich, according to The Associated Press. Another 57 delegates are up for grabs in Indiana, a state that has voted Republican in nine of the last 10 presidential elections.

Top Trump aide Corey Lewandowski told CNN on Tuesday the campaign expected to win more than required number of delegates - 1,300 to 1,400.

Cruz vowed on Monday to "compete to the end" but a loss in Indiana would be particularly crushing for the senator, who has argued that his brand of religious conservatism is a natural draw for heartland Republicans. He won the endorsement of conservative Indiana Governor Mike Pence.

Cruz had hoped for smoother sailing in Indiana after he and John Kasich reached a "stop-Trump" deal in which Kasich would steer clear of the state while Cruz would do likewise in Oregon and New Mexico.

But the waters are looking choppier for Cruz, with the senator losing considerable ground against Trump in opinion polls as voting has neared.

Cruz last week also announced his choice for a prospective vice president, the former presidential contender and Hewlett Packard Chief Executive Carly Fiorina, during an event in Indiana that some criticized as premature.

Trump has drawn both passionate support and vitriolic condemnation with his stands on immigration and national security - including a call to build a 1,000-mile wall along the Mexican border that he says Mexico would pay for and a bid to temporarily ban Muslims from entering the country.

Julie Blackwell Chase, a clerk treasurer in the town of Bedford in southern Indiana, said she voted early for Trump in part because she appreciated his willingness to break with conventional politics. "We need new blood," she said.

The outcome in Indiana may ride on the votes of evangelical Christians after Trump offered praise for Planned Parenthood family clinics and signaled support for gay and transgender rights - views that rankled some Christian conservatives.

Clark County Sheriff Jamey Noel, chair of the county's Republican Party, said he is voting for Cruz.

"Traditional Republicans and Republicans who understand how we elect our president, or pretty much everyone who's conservative, likes Cruz," he said.

Jeff Cardwell, chairman of the state's Republican Party, said Tuesday's primary marked an exciting day in Indiana politics, mainly because White House nominations are usually locked up by this late in the election cycle.

"This is the first time in my lifetime where Indiana has really had an opportunity to make a difference," said.

Indiana has the second-largest delegate haul of the 10 states remaining in the 2016 Republican nominating contest, behind California, which holds its primary June 7.

On the Democratic side, front-runner Hillary Clinton holds a more than 6 percentage point lead over challenger Bernie Sanders, a US senator from Vermont, according to an average of recent polls compiled by Real Clear Politics.