This story has been updated with details about David Duke and other white supremacist support of Trump's campaign, and to add comment from the Democratic candidates.
In an interview Sunday morning, GOP front-runner Donald Trump would not condemn former Ku Klux Klan grand wizard David Duke's support for his presidential campaign, telling CNN host Jake Tapper on "State of the Union" that he has no knowledge of the white supremacist leader.
"Just so you understand, I don't know anything about David Duke, okay? I don't know anything about what you're even talking about with white supremacy or white supremacists. So I don't know, did he endorse me or what's going on," Trump said.
When Tapper asked if Trump would unequivocally condemn and reject the white supremacists who support him, Trump said he would need to conduct research into the groups.
"Well, I have to look at the group. I mean, I don't know what group you're talking about. You wouldn't want me to condemn a group that I know nothing about. I have to look. If you would send me a list of the groups I will do research on them and certainly I would disavow if I thought there was something wrong. But you may have groups in there that are totally fine and it would be very unfair. So give me a list of the groups and I'll let you know," Trump said.
Tapper clarified that he was referring to Duke and the Ku Klux Klan, to which Trump again insisted that he was unfamiliar with the white supremacist leader.
"Honestly, I don't know David Duke. I don't believe I've ever met him. I'm pretty sure I didn't meet him. And I just don't know anything about it," Trump said.
Duke made headlines this week when he told the audience of his radio program that voting for anyone besides Trump “is really a treason to your heritage.” BuzzFeed News first reported his comments.
“When this show’s over, go out, call the Republican Party, but call Donald Trump’s headquarters, volunteer,” Duke said. “They’re screaming for volunteers. Go in there, you’re gonna meet people who are going to have the same kind of mindset that you have.”
Duke also told Politico in December that Trump’s presidential bid permits Americans to be more open about racial hostility, saying, “He’s made it okay to talk about these incredible concerns of European Americans today, because I think European Americans know they are the only group that can’t defend their own essential interests and their point of view.”
Despite this morning's interview, Trump has not always claimed ignorance of Duke and his involvement with the KKK. During an August 2015 interview with Bloomberg News, the business mogul said that he did not want Duke's endorsement, and that he disavowed him. Going even further back, Trump expressed disapproval of Duke in 2000 after choosing not to explore on a presidential bid in the Reform Party.
“The Reform Party now includes a Klansman, Mr. Duke, a neo-Nazi, Mr. Buchanan, and a communist, Ms. Fulani,” Mr. Trump said in an statement reported by the New York Times in 2000. “This is not company I wish to keep.”
Trump's rivals condemned his statements shortly after the interview with CNN aired. Sen. Marco Rubio argued on the campaign trail that the GOP could not grow with a candidate like Trump.
"We cannot be a party that nominates someone who refused to condemn white supremacists and the Ku Klux Klan," Rubio told a fired-up crowd in Purcellville, Virginia.
Duke is not the only white supremacist who has advocated for Trump. William Daniel Johnson of the white nationalist American Freedom Party contributed financially to the Trump campaign, which said it would return the donation. Johnson also founded a super PAC that put out pro-Trump robocalls in early voting states.
“We don’t need Muslims. We need smart, well-educated white people,” a voice said in one of the New Hampshire robocalls. “I am a farmer and a white nationalist. Support Donald Trump,” said another.
Rubio said Trump's refusal to disavow hate groups "makes him unelectable" and would prevent the party from reaching out to new voters.
Other GOP candidates, Sen. Ted Cruz and Gov. John Kasich, blasted Trump on Twitter.
When asked about his father's refusal to repudiate the support of white supremacists, Donald Trump Jr., Trump's son and executive vice president of the Trump Organization, told MSNBC's Chris Jansing that while he was not a spokesman for the campaign, he was a spokesman for his father — and that "I'm pretty sure we're not interested in those kinds of votes."
"So, you're willing to say that you do not want the support of a former grand wizard of the Ku Klux Klan?" Jansing asked.
"Yeah, I'm saying that," Trump Jr. said.
Later, the presidential candidate took to Twitter to share a video of a press conference he held last Friday. In regard to David Duke's support, he wrote, "I disavow."
Meanwhile, Trump's latest controversy briefly united Democratic candidates Hillary Clinton and Bernie Sanders in the midst of their battle to win over Super Tuesday voters.
Hillary Clinton's Twitter account agreed, and retweeted the senator's account.
Benjy Sarlin contributed to this report.