The Republican Party began to fracture over its presidential front-runner, Donald Trump, as Nebraska Senator Ben Sasse vowed never to support the billionaire and Mitt Romney said his remarks over the weekend about the Ku Klux Klan were "disqualifying and disgusting."

As he gains in the polls, Trump has gathered endorsements by other prominent Republicans, including New Jersey Governor Chris Christie, former Arizona Governor Jan Brewer, and Alabama Senator Jeff Sessions. But in television interviews Sunday and Monday, Trump declined to immediately repudiate support for his campaign by David Duke, a former KKK leader.

Trump has blamed a faulty television earpiece for his uncertain answers on Sunday to questions on CNN about Duke and the KKK.

Romney, the former Massachusetts governor who was the Republican presidential candidate in 2012, called Trump’s remarks on the Klan “a disqualifying & disgusting response” in a post Monday on Twitter. “His coddling of repugnant bigotry is not in the character of America,” Romney said.

Senator Marco Rubio, a Republican presidential candidate who has been trading insults with Trump in recent days, mocked Trump’s explanation of his answers to the Duke and KKK questions.

No matter how badly the earpiece was working, "‘Ku Klux Klan’ comes through pretty clearly," Rubio said during a campaign event, according to the Associated Press.

Disavowing Duke

Trump said Monday in an interview on NBC’s “Today Show” that he has never met white supremacist Duke, who spoke favorably of Trump on his radio program recently and praised him for “taking on the Jewish establishment.”

“I know who he is, but I never met David Duke,” Trump said Monday when asked about his comments on Sunday. “I disavowed David Duke the day before.”

Trump said he thought he was being asked by CNN host Jake Tapper on Sunday about Duke and “various groups." Trump said he wanted to know which groups.

“I’m sitting in a house in Florida with a very bad earpiece,” Trump explained Monday about the circumstances of the CNN interview. “What I heard was ‘various groups.”’

Speaking later Monday at a campaign rally in Radford, Virginia, Trump was interrupted by protesters who support the Black Lives Matter movement. He responded to them by saying, “All lives matter.”

Sessions said on a radio program Monday in Alabama that Trump should demonstrate support for racial equality. “I hope he makes that clear,” Sessions said on “The Matt Murphy Show.”

‘Not our party’

South Carolina Governor Nikki Haley, a Republican who has endorsed Rubio, criticized Trump for his comments.

“I will not stop fighting a man who refuses to disavow the KKK,” she said Monday in Atlanta. “That’s not who we are and not our party.”

White House press secretary Josh Earnest, who has fielded questions for months about Trump’s candidacy, said Trump must decide if he has sufficiently distanced himself from Duke.

“I know Mr. Trump says that there is more that he needs to learn about Mr. Duke” to render his opinion, Earnest said at the White House. "We know all that we need to know about Mr. Trump.”

Trump also said Monday he will fight back against the Republican establishment if the party works against him as he leads in delegates to secure the presidential nomination.

The Pledge

“If they violate the pledge, I will do something that will make them very unhappy,” Trump said. Trump signed a pledge last year for the Republican National Committee in which he agreed not to run as a third-party candidate. The party agreed to maintain a level playing field in the race.

Sasse, a freshman senator, said on his personal Twitter account Sunday, “If Trump becomes the Republican nominee my expectation is that I’ll look for some 3rd candidate – a conservative option, a Constitutionalist.”

A nationwide CNN/ORC poll released Monday found Trump had 49 percent support among Republicans. Rubio had 16 percent and Senator Ted Cruz of Texas had 15 percent.

Trump said the lower poll numbers for Rubio explain the Florida senator’s recent change in tone. In the past few days, the two have traded insults on the campaign trail.

“He did it because he’s desperate,” Trump said. “He’s down in the polls. He’s a nervous wreck.”

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