MADISON, Ala. — The implosion over Donald Trump's candidacy that Republicans had hoped to avoid arrived so virulently this weekend that many party leaders vowed never to back the billionaire and openly questioned whether the GOP could come together this election year.

At a moment when Republicans had hoped to begin taking on Hillary Clinton — who is seemingly on her way to wrapping up the Democratic nomination — the GOP has instead become consumed by a crisis over its identity and core values that is almost certain to last through the July convention, if not the rest of the year.

A campaign full of racial overtones and petty, R-rated put-downs grew even uglier Sunday after Trump declined repeatedly in an interview on CNN's “State of the Union” to repudiate the endorsement of him by David Duke, a former grand dragon of the Ku Klux Klan. Trump had disavowed Duke at a news conference on Friday, but he stammered when asked about Duke on Sunday, who last week told his radio listeners that a vote against Trump was equivalent to “treason to your heritage.”

“Well, just so you understand, I don't know anything about David Duke. OK?” Trump said. “I don't know anything about what you're even talking about with white supremacy or white supremacists.”

Trump's comments came the same day he retweeted a quote from Benito Mussolini, the 20th-century fascist dictator of Italy.

Marco Rubio, who has been savaging Trump as a “con man” for three days, responded by saying that Trump's defiance made him “unelectable.” The senator from Florida said at a rally in northern Virginia, “We cannot be the party that nominates someone who refuses to condemn white supremacists.”

Texas Sen. Ted Cruz called Trump's comments “really sad.”

“You're better than this,” Cruz wrote on Twitter. “We should all agree, racism is wrong, KKK is abhorrent.”

Trump was asked Friday by journalists how he felt about Duke's support. He said he didn't know anything about it and curtly said: “All right, I disavow, OK?”

The billionaire hasn't always claimed ignorance on Duke's history. In 2000, he wrote a New York Times op-ed explaining why he abandoned the possibility of running for president on the Reform Party ticket. He wrote of an “underside” and “fringe element” of the party, concluding, “I leave the Reform Party to David Duke, Pat Buchanan and Lenora Fulani. That is not company I wish to keep.”

“This is an existential choice,” said former Minnesota Sen. Norm Coleman, who is backing Rubio. Asked how the party could unite, Coleman said, “It gets harder every day when you hear things like not disavowing the KKK and David Duke. It's not getting easier; it's getting more difficult. ... I'm hopeful the party won't destroy itself.”

The choice for voters is not simply one of preference but rather a fundamental one about the direction they want to take the country, with the insurgent Trump promising utter transformation.

“For many Republicans, Trump is more than just a political choice,” said Kevin Madden, a veteran operative who advised 2012 nominee Mitt Romney. “It's a litmus test for character.”

Madden, like some of his peers, said he could never vote for Trump. If he is the nominee, Madden said, “I'm prepared to write somebody in so that I have a clear conscience.”

Some Republican leaders, however, are making far different calculations. New Jersey Gov. Chris Christie enthusiastically endorsed Trump on Friday, and the two campaigned together across the South for two days.

But directly associating himself with Trump has been problematic for Christie. He stumbled through an interview Sunday with ABC's George Stephanopoulos as the anchor questioned him over Trump's proposed ban on Muslims entering the country.

Christie also drew public scorn from one of his top backers, Hewlett Packard Chief Executive Meg Whitman, who issued a scathing statement condemning Christie for an “astonishing display of political opportunism” and calling Trump “a dishonest demagogue” who would “take America on a dangerous journey.”

Nonetheless, Trump appeared to continue to steamroll toward Super Tuesday. He staged a massive rally at a football stadium in Madison, Ala., where he received the endorsement of Sen. Jeff Sessions of Alabama, a Capitol Hill veteran and a nationally recognized opponent of illegal immigration.

“I told Donald Trump, ‘This isn't a campaign; this is a movement,'?” Sessions exhorted.

“You have asked for 30 years and politicians have promised for 30 years to fix illegal immigration. Have they done it?” Sessions asked the crowd.

“No!” thousands shouted back.

“Donald Trump will do it,” Sessions promised.

Pointing to his latest endorsements, Trump told the raucous Alabama crowd, “I hate to say it, but I'm becoming mainstream.”

Associated Press contributed.