Follow CBSMIAMI.COM: Facebook | Twitter

INDIANAPOLIS (CBSMiami/AP) — The race for the White House took a bizarre turn Tuesday when Republican front-runner Donald Trump tied Ted Cruz’s father to the man who assassinated President John F. Kennedy.

READ MORE: Feds Pay First Florida School District For COVID-19 Mask Mandate

Texas Sen. Cruz, who is facing a critical moment for his struggling campaign, unleashed a blistering attack against Trump, calling the businessman “amoral” and warning the country could “plunge into the abyss” if he is elected president.

Trump, growing increasingly confident about his chances of clinching the GOP nomination, responded by saying Cruz “does not have the temperament to be president of the United States.” Earlier Tuesday Trump had rehashed unsubstantiated claims that the Texan’s father, Rafael Cruz, appeared in a 1963 photograph with Kennedy’s assassin, Lee Harvey Oswald — citing a report first published by the National Enquirer.

“His father was with Lee Harvey Oswald prior to Oswald’s being, you know, shot,” Trump said on Fox & Friends. “Nobody even brings it up; I mean they don’t even talk about that.”

The heated exchanges come as Cruz’s opportunities to block Trump dwindle. While Indiana’s primary once looked like a ripe opportunity for the conservative Texas senator to make up ground, his campaign has faltered here and aides were pessimistic about their prospects.

During one of his final stops in the state, a visibly exasperated Cruz let his frustrations with Trump fly, calling him a “pathological liar,” a “serial philanderer,” ”kooky,” ”nuts” and “terrified with strong women.”

Hillary Clinton and Vermont Sen. Bernie Sanders also faced off in Indiana’s Democratic primary on Tuesday, though the stakes were lower than in the Republican race. Clinton holds a commanding lead, with 91 percent of the delegates she needs to win the nomination. That means she can still win even if she loses every remaining contest.

Sanders has conceded that he faces a difficult path, one that hinges on persuading superdelegates to back him over the former secretary of state. Superdelegates are Democratic Party insiders who can support the candidate of their choice, regardless of how their states vote. And they favor Clinton by a nearly 18-1 margin.

Neither Clinton nor Sanders were in Indiana Tuesday. Sanders was making stops in Kentucky, which holds a primary in mid-May, while Clinton moved on to Ohio, a key general election battleground.

A fall showdown between Clinton and Trump would pit one of Democrats’ most experienced political figures against a first-time candidate who is deeply divisive within his own party. Cruz and other Republicans have argued that Trump would be roundly defeated in the general election, denying their party the White House for a third straight term.

READ MORE: Miami Weather: Partly Cloudy With Scattered Afternoon Storms Through Saturday, Cold Front Moves In Sunday

Still, Trump has won six straight primary contests and has 80 percent of the delegates needed to secure the GOP nomination. Cruz, as well as Ohio Gov. John Kasich, can only hope to keep him from the 1,237 delegates he needs and push the GOP race to a contested convention.

Cruz has spent the past week camped out in Indiana, securing the support of the state’s governor and announcing Carly Fiorina, the retired technology executive, as his running mate. He’s vowed to stay in the race as long as he has a possible path.

Trump, too, devoted more time to campaigning in Indiana than he has to most other states, underscoring his eagerness to put his Republican rival away and shift his attention toward Clinton.

While Trump cannot clinch the nomination with a win in Indiana, his path would become easier and he would have more room for error in the campaign’s final contests. After Cruz’s comments in Evansville, he said in a statement the Texas senator was “a desperate candidate trying to save his failing campaign.”

Cruz said America is “looking, potentially, at the Biff Tannen presidency,” referencing a character in the “Back to the Future” films. He described the character as “a braggadocious, arrogant buffoon who builds giant casinos with giant pictures of him everywhere he looks.”

The film’s screenwriter Bob Gale told the Daily Beast last year that the character was based on Trump.

Cruz’s aides were cautious heading into Tuesday’s vote. Campaign officials had been told to prepare for Cruz to deliver “a very somber” speech Tuesday night in Indianapolis, according to one aide who spoke on the condition of anonymity to share internal discussions.

Republican leaders spent months dismissing Trump as little more than an entertainer who would fade once voting started. But Republican primary voters have stuck with the billionaire businessman, handing him victories in every region of the country.

For more on Campaign 2016, click here.

MORE NEWS: Tracking The Tropics: Hurricane Sam Rapidly Strengthening In Atlantic

(TM and © Copyright 2016 CBS Radio Inc. and its relevant subsidiaries. CBS RADIO and EYE Logo TM and Copyright 2016 CBS Broadcasting Inc. Used under license. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)