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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 party 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 a CNN interview to repudiate the endorsement of him by David Duke, a former grand wizard of the Ku Klux Klan. Trump had disavowed Duke at a news conference on Friday, but he stammered when asked about Duke on Sunday.

During appearances on network television Feb. 28, Republican presidential candidate Donald Trump repeatedly declined to refuse the endorsement of David Duke, a former grand wizard of the Ku Klux Klan. While Marco Rubio and Ted Cruz both took aim at Trump. (The Washington Post)

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.”

The fracas comes as the presidential race enters a potentially determinative month of balloting, beginning with primaries and caucuses in 11 states on Tuesday. As the campaign-trail rhetoric grew noxious over the weekend, a sense of fatalism fell over the Republican firmament, from elected officials and figureheads to major donors and strategists.

“This is an existential choice,” said former senator Norm Coleman of Minnesota, 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.

[Where Trump could lose, and other things to watch for on Super Tuesday]

“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.”

More splintering came late Sunday when freshman Sen. Ben Sasse (R-Neb.), who has been a vocal Trump critic, declared on Twitter that if the reality TV star is nominated, he will “look for some 3rd candidate — a conservative option, a Constitutionalist.”

[The overlooked victim of Trump’s campaign? Martin Van Buren]

Some Republican leaders, however, are making far different calculations. New Jersey Gov. Chris Christie enthusiastically endorsed Trump on Friday, and the two looked like running mates as they 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, 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.

Sen. Jeff Sessions (R-Ala.) endorsed Republican presidential candidate Donald Trump's White House bid during a joint appearance in his home state. (Reuters)

“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.”

This is not how Republican officials imagined their party would be entering the spring of 2016. They had wanted to unite around a nominee with an inclusive and broadly appealing message and begin prosecuting the case against Clinton.

Instead, they are wondering anew whether mainstream voters could accept Trump as the nominee.

“It’s scary,” South Carolina Gov. Nikki Haley, who has endorsed Rubio, said on ABC’s “This Week.” She added: “I think what he’ll do to the Republican Party is really make us question who we are and what we’re about. And that’s something we don’t want to see happen.”

Then there’s the question of whether Trump’s fiercely loyal base of backers would shift their allegiance to Rubio or one of the other candidates — Sen. Ted Cruz of Texas, Ohio Gov. John Kasich or retired neurosurgeon Ben Carson — were Trump to lose the nomination.

Rubio and his aides have been promoting a #NeverTrump campaign on Twitter.

[Spray tans, pants-wetting and little hands: The GOP race goes in the gutter]

Trump said Sunday that the opposition was the latest slight against him from party insiders and a “total violation” of the Republican National Committee pledge each candidate signed vowing to support the party’s eventual nominee.

“I’m representing a lot of anger out there,” Trump said on CNN’s “State of the Union.” “We’re not angry people, but we’re angry at the way this country’s being run. And a lot of them are angry at the way the Republican Party is being run.”

With Rubio vowing never to quit the race and preparing to fight Trump and possibly other candidates for delegates at a brokered convention, Trump’s allies are trying to master party rules and are laying plans to organize delegates.

Roger Stone, a Republican consultant and longtime Trump associate who does not work for the campaign, said Sunday that he has been reaching out to fellow alumni of Ronald Reagan’s presidential campaign about volunteering on Trump’s behalf at the convention.

“We’re looking at pulling together people with deep knowledge and experience at Republican conventions going back to Barry Goldwater in ’64 or Reagan in ’76,” Stone said of his efforts, which have been taking place by telephone. “If they try to change the rules to steal it from Trump, he’ll need friends who understand the rules.”

Some party leaders are openly wondering how Rubio, after labeling Trump a “con man,” could show up at the convention in Cleveland and endorse him.

“I’m not sure that he can — or that he’d be invited, for that matter,” said Trent Lott, a former Senate Republican leader from Mississippi who is backing ­Kasich. “It won’t be easy to get all the forces back together.”

But Lott added: “I don’t think people have any idea what Trump would do. He might wind up being the most magnanimous, inviting and generous person you could imagine. Who knows?”

The challenging task of uniting the party falls to the RNC, which oversees the convention. Sean Spicer, the committee’s chief strategist, said the prospect of a Clinton presidency would serve as a unifying force.

“There is an overwhelming understanding in our party that we have to be united against Hillary Clinton, because there is too much at stake, if you just look at the Supreme Court alone,” Spicer said. “After the last eight years, everyone on the Republican side understands that.”

Richard Wadhams, a former chairman of the Colorado Republican Party, said there has been a growing acceptance of Trump in recent weeks among party leaders and rank-and-file activists alike.

“There’s a strong possibility that Trump is going to be the nominee, and a lot of Republicans are ready to accept that even though they’ve not been supporters of him,” Wadhams said. “The penchant to defeat Hillary Clinton will transcend any concerns about the way Trump has conducted himself.”

Costa reported from Atlanta. Jose A. DelReal in Madison contributed to this report.