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INDIANAPOLIS — Senator Ted Cruz, who for months last year embraced Donald J. Trump as a force for good in the Republican presidential race, unburdened himself as never before on Tuesday in a searing, personal barrage hours before the Indiana primary.

“I’m going to do something I haven’t done for the entire campaign,” he told reporters in Evansville, Ind. “I’m going to tell you what I really think of Donald Trump.”

Mr. Trump is a “pathological liar,” he said — an “utterly amoral” man and a “narcissist at a level I don’t think this country has ever seen.”

He attacked Mr. Trump’s history of adultery, his penchant for conspiracy theories and his past disparaging Twitter posts about Heidi Cruz, Mr. Cruz’s wife.

“Apparently she’s not pretty enough for Donald Trump,” Mr. Cruz said, with Mrs. Cruz smiling beside him. “I may be biased, but I think if he’s making that allegation he’s also legally blind.”

In a statement, Mr. Trump called Mr. Cruz “a desperate candidate trying to save his failing campaign” and growing “more and more unhinged” amid a string of primary losses.

Though Mr. Cruz has long attacked Mr. Trump as a disingenuous conservative and unsteady leader — particularly as he has strained to catch Mr. Trump in the Indiana polls and rescue a flagging presidential bid — his performance on Tuesday was different.

Hours earlier, Mr. Trump had said in a Fox News interview that Mr. Cruz’s father Rafael had associated with Lee Harvey Oswald not long before the assassination of John F. Kennedy. He cited a story from the National Enquirer, which the Cruz campaign has called error-filled.

“I guess I should go ahead and admit, yes, my dad killed J.F.K.,” Mr. Cruz said dryly. “He is secretly Elvis and Jimmy Hoffa is buried in his backyard.”

Repeatedly, he pleaded with voters in Indiana to think carefully about their choices, at times calling to mind the final campaign days of Senator Marco Rubio of Florida, who railed against Mr. Trump with uncommon purpose as his prospects waned.

He noted that the office of the president “affects our culture,” setting an example for children.

“Listen, Donald Trump is a serial philanderer and he boasts about it,” Mr. Cruz said, directly raising Mr. Trump’s marital history for the first time. “I want everyone to think about your teenage kids. The president of the United States talks about how great it is to commit adultery. How proud he is. Describes his battles with venereal disease as his own personal Vietnam.”

He went on to accuse the news media of enabling Mr. Trump, singling out Rupert Murdoch and Roger Ailes of Fox News for transforming the network into “the Donald Trump network, 24/7.”

The most surreal turn came perhaps near the end, when Mr. Cruz, a movie buff, reached for an analogy to cinema.

“If anyone has seen the movie ‘Back to the Future 2,’” he began, “the screenwriter says that he based the character Biff Tannen on Donald Trump, a caricature of a braggadocious arrogant buffoon.”

“We are looking, potentially,” he added sternly, “at the Biff Tannen presidency.”

Despite public vows to remain in the race regardless of Tuesday’s outcome, Mr. Cruz again implored Indiana voters to help.

“If Indiana does not act,” he said, “this country could well plunge into the abyss.”

As he neared the end of his remarks, in his final scheduled appearance before the vote, Mr. Cruz was asked if he could still support Mr. Trump as the Republican nominee.

For weeks, he has wavered, after initially pledging to support any eventual nominee. At last, it seemed possible that he would make his opposition unequivocal.

He declined to answer.