Donald Trump has had better Sunday mornings. The Republican presidential front-runner stepped in controversy when he repeatedly declined to repudiate white supremacist support Sunday, passing up a chance to reject backing from former Ku Klux Klan leader David Duke. The GOP front-runner said he was unfamiliar with white supremacist groups, and said he would not specifically repudiate them before researching them.

At a press conference Friday, Trump did reject Duke's endorsement, however, and Newt Gingrich later came to Trump's defense on Twitter.

Trump rival Sen. Ted Cruz also made the rounds Sunday, speculating about Trump's ties to the mafia, but agreeing to back the GOP front-runner in a general election.

Meanwhile, Trump backer Chris Christie defended his endorsement of the real estate developer.

Trump won't denounce KKK supporter David Duke's endorsement: Trump refused to condemn racist groups that would support his candidacy, a few days after former Ku Klux Klan member David Duke endorsed him.

"I have to look at the group," Trump told Jack Tapper on CNN's "State of the Union" Sunday.

"If you would send me a list of the groups I would do research on them and I would disavow," Trump said.

Gossip site Gawker took credit Sunday for tricking Trump into retweeting a statement by Mussolini. Looking flummoxed when confronted with his accidental endorsement of "Il Duce" on NBC's "Meet the Press," Trump channeled Hillary Clinton. "What difference does it make whether it was Mussolini or someone else?" he asked.

Haley: Trump is not a good role model for your children: South Carolina Gov. Nikki Haley described Donald Trump as a bad model for children, unlike Florida Sen. Marco Rubio, who she supports. "Donald Trump is everything we hear and teach our kids not to do in kindergarten," Haley told ABC News' George Stephanopoulos on Sunday. "We have seen this behavior over and over again that's unacceptable."

Cruz will support GOP nominee, even Trump: Texas Sen. Ted Cruz will support Donald Trump if he becomes the GOP nominee, but it appears he won't be happy about it. "We need a candidate who can beat Hillary," Cruz, who trails the real estate developer in most polls as they fight for the Republican presidential nomination, said on CNN's "State of the Union" Sunday.

Sanders: 'We got killed' in South Carolina: Bernie Sanders didn't mince words about his huge South Carolina primary loss to Hillary Clinton. "We got decimated," Sanders told ABC News' George Stephanopoulos on Sunday, one day after he lost by nearly 50 percent in the Palmetto State. "The only positive thing for us is we won the actually, the 29 years of age and younger vote. And that was good," Sanders added. "But we got killed."

Post-endorsement Christie doesn't see eye to eye with Trump: New Jersey Gov. Chris Christie began the tricky task Sunday of explaining his endorsement of a candidate whose proposals he roundly rejected just weeks ago. During an interview on ABC News' "This Week" Christie defended Donald Trump, despite their differences, as the best presidential candidate available. During his own campaign, Christie said Trump's plan to build a wall along the southern border and have Mexico pay for it "makes no sense." Christie danced around the issue Sunday. "I'm not going to support everything that happens in any campaign," Christie said.

Cruz speculates about Trump ties to mob: Sen. Ted Cruz speculated that Donald Trump is not releasing his tax returns because it may reveal potential ties to organized crime in New York. "We don't know what it is he is hiding in those tax returns," said Cruz on "Meet the Press" Sunday. Cruz released nine years of returns on Saturday, and Marco Rubio released five years' worth of returns.

Christie: I didn't tell publisher I would never endorse Trump: "Some of your supporters say you promised them you would not endorse Donald Trump," "This Week" host George Stephanopoulos told Christie during an interview Sunday. "Joe McQuaid, the publisher of the New Hampshire Union Leader, says you promised him on a phone call you wouldn't endorse Donald Trump." "It's just not true," Christie replied. "He called me two days after the primary and said, 'I was just told that you're about to endorse Donald Trump.' And I said to him, 'that's absolutely untrue. I'm not about to endorse anybody.'"

Cruz: Trump will 'bargain away' rights: "We've had too many deals in Washington. It's why we're bankrupting our kids and grandkids," Cruz told ABC's George Stephanopoulos on Sunday. "I am not going to compromise on the Bill of Rights," Cruz said. "I'm not going to bargain away your religious liberty. I'm not going to bargain away your Second Amendment rights and sit down with [Senate Minority Leader] Harry Reid or [New York Sen.] Chuck Schumer and agree on Supreme Court nominees," Cruz said.