Trump and Cruz
Trump and Cruz Reuters

Republican presidential frontrunner Donald Trump was heavily criticized by his rivals on Sunday over his initial refusal to disavow an endorsement from David Duke, the former leader of the Ku Klux Klan, who announced his support for Trump late last week, while warning supporters against supporting any other candidates.

Trump responded to the news of Duke's endorsement by telling a press conference on Friday he "disavowed" it, but on Sunday, in an interview on CNN, he refused to do so again.

Pressed by CNN's Jake Tapper about whether he would disavow Duke and other white supremacist groups that are supporting his campaign, Trump replied, "Just so you understand, I don't know anything about David Duke, OK?"

Trump was pressed three times on whether he'd distance himself from the Ku Klux Klan but never mentioned the group in his answers.

"I don't know anything about what you're even talking about with white supremacy or white supremacists," he told Tapper. "So I don't know. I don't know -- did he endorse me, or what's going on? Because I know nothing about David Duke; I know nothing about white supremacists."

Later on Sunday, after his CNN appearance, Trump posted a clip on Twitter from Friday's news conference, confirming he does indeed disavow Duke.

“As I stated at the press conference on Friday regarding David Duke- I disavow,” tweeted Trump.

But even as he disavowed Duke's endorsement a second time, Ted Cruz and Marco Rubio, two of Trump's main rivals in the Republican race, took advantage of the situation to blast him.

“We cannot be a party who refuses to condemn white supremacists and the Ku Klux Klan,” Rubio told thousands of supporters in Leesburg, Virginia on Sunday.

“Not only is that wrong, it makes him unelectable. How are we going to grow the party if we nominate someone who doesn’t repudiate the Ku Klux Klan?” added the Florida senator.

Cruz blasted Trump as well, taking to Twitter and calling Trump’s comments “really sad.”

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

Another Republican presidential candidate, John Kasich, weighed in as well and wrote on Twitter: "Hate groups have no place in America. We are stronger together. End of story."

The criticism of Trump also naturally moved over to the Democratic side, where Senator Bernie Sanders tweeted, “America’s first black president cannot and will not be succeeded by a hatemonger who refuses to condemn the KKK.”

And Sanders' rival, Hillary Clinton, did her part by retweeting Sanders' message.