INDIANAPOLIS – Republican Ted Cruz launched a blistering attack on rival Donald Trump as voters cast ballots in Indiana on Tuesday, calling him “amoral” and likening him to a fictitious character he described as “a braggadocious, arrogant buffoon.” Replied Trump, “Cruz does not have the temperament to be president of the United States.”

Speaking to reporters in Evansville on the day of the state’s presidential primary, Cruz warned that the country could “plunge into the abyss” if Trump is elected president.

“We are not a proud, boastful, self-centered, mean-spirited, hateful, bullying nation,” Cruz said, with his wife Heidi and running mate Carly Fiorina by his side.

Cruz faces a high-stakes test of his presidential campaign in the primary, one of the last opportunities for the Texas senator to halt Trump’s stunning march toward the GOP nomination.

Cruz has spent the past week camped out in Indiana, securing the support of the state’s governor and announcing Fiorina, a retired technology executive, as his running mate.

“I am in for the distance, as long as we have a viable path to victory,” Cruz he said Monday.

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 Democratic front-runner Hillary Clinton.

BIG WIN WON’T CLINCH NOMINATION

While Trump cannot clinch the nomination with a big win in Indiana, his path would get 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 is “a desperate candidate trying to save his failing campaign.”

“It is no surprise he has resorted to his usual tactics of over-the-top rhetoric that nobody believes,” Trump said.

Earlier in the day, Trump had launched his own tirade against Cruz, rehashing unsubstantiated claims that Cruz’s father, Rafael Cruz, appeared in a 1963 photograph with John F. 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 Tuesday on Fox News. “Nobody even brings it up; I mean they don’t even talk about that.”

Cruz responded to the claims by calling his father his “hero” and labeling Trump as “amoral.”

Cruz also 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.

PREPARING FOR ‘SOMBER SPEECH’

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, including a string of six straight wins on the East Coast.

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 – she’s secured 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 to overtake Clinton, 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 planned to spend Tuesday in Indiana. 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 showdown between Clinton and Trump would pit one of Democrats’ most popular and highly regarded 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.

Peoples reported from Washington. Associated Press writer Stephen Ohlemacher contributed to this report from Washington.