Donald Trump has accused his Republican presidential nomination rival Ted Cruz's father of helping in the assassination President John F Kennedy, prompting Cruz to call the billionaire "a pathological liar".
Trump, who looks set to win the Indiana primary and solidify his lead despite having never held political office, was referring to a debunked story in The National Enquirer that claimed Rafael Cruz had associated with JFK assassin Lee Harvey Oswald before the November, 1963 killing.
The row with Cruz comes as Trump is predicted to win Tuesday's Indiana poll, victory in which will mean the race is "over", Trump himself has said.
In a campaign already defined by Trump's absurd claims and accusations, this one triggered fury from the Cruz campaign, which condemned the story and the billionaire candidate with strong words.
The tabloid claimed it had analysed an photo of Oswald, taken months before he shot the president, and found another man in the photo, who was never identified, is Rafael Cruz.
Ted Cruz's campaign has said the man in the picture is someone else.
A spokeswoman for his campaign told The Miami Herald: "It's embarrassing that anyone would enable Trump to discuss this.
"It's a garbage story and clearly Donald wants to talk about garbage. Ted Cruz will do what he's been doing, talking about jobs, freedom, and security for the American people."
In the contest for the most surreal theory about Cruz, this one is competing with claims he is the Zodiac Killer and claims he is a lizard in a human costume.
Trump ranted about the story during a phone-in interview with Fox News on Tuesday morning.
"His father was with Lee Harvey Oswald prior to Oswald's being — you know, shot. I mean, the whole thing is ridiculous," he said.
"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? It’s horrible."
When confronted with the claim, Cruz joked: "Yes, my dad killed JFK. He's secretly Elvis and Jimmy Hoffa is buried in his backyard."
He told reporters: "I'm gonna do something I haven't done for the whole campaign ... I'm gonna tell you what I really think about Donald Trump."
"[Trump] doesn't know the differences between truth and lies. He lies practically every word that comes out of his mouth," Cruz added.
"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's ever seen."
He said Trump's persistent accusations that the other candidates were liars were "something straight out of a psychology textbook".
Pastor Rafael Cruz, who to went to America from Cuba in the 1950s, has previously appealed for Christians to vote his evangelical son, saying a vote for anyone else could mean "the destruction of America".
He said: "I implore every member of the Body of Christ to vote according to the word of God. And vote for the candidate that stands for the word of God and the Constitution of the United States of America… The alternative could be the destruction of America".
But Trump is forecast to be well ahead in Indiana. A HuffPost poll puts him on 43 percent and Cruz on 29 percent.
A victory there would boost his momentum that would make him almost certainly his party's nominee, unless there were a dramatic upset later on.
Cruz's attack on Trump follows a long history of Trump insulting him throughout the campaign, calling him "lyin' Ted".
In a desperate move, Cruz announced his running mate - hifghly unusual for a candidate who has not yet won the nomination - as former candidate Carly Fiorina.
Trump is well ahead in the delegates - 996 to Cruz's 565 in second place - and he needs 1,237 to win.
Ahead of the intensely-fought battle in Indiana, Trump said: "If we win Indiana, it’s over – they’re finished, they are done."