Rep. Peter King says Donald Trump is "genuinely dumb" if he stands by statements he made Sunday about the recent endorsement Trump received from the former leader of the KKK.

The business mogul and Republican front-runner was endorsed by former KKK "grand wizard" David Duke. Appearing on CNN's "State of the Union" with Jack Tapper Sunday, Trump wouldn't condemn the endorsement because he said he doesn't know anything about Duke. Trump said that if he were "sent a list of groups" he would research them, and said he would disavow any if he thought there was "something wrong."

"You wouldn't want me to condemn a group that I know nothing about," Trump said.

"Donald Trump's refusal to denounce and disavow support from KKK supporter David Duke and white supremacist organizations is indefensible and disgraceful," Rep. King said in a statement. "Trump told Jake Tapper he doesn't know who David Duke is or what white supremacist organizations stand for. If Trump's statement is true, then he is genuinely dumb. If he is lying, that is shameful. In either case, he should not be running to lead the United States."

King's statement continued, "As a Catholic I am particularly disgusted because of the KKK's long history and record of vicious anti-Catholicism. And as someone who grew up in Queens, I can say that, unlike Trump, real tough guys aren't afraid to take on the KKK."

King last week announced his endorsement of Marco Rubio for the presidency, saying Rubio is "the adult when it comes to foreign policy and homeland security."

Hours after Trump made the comments on CNN, he tweeted a video clip of an appearance he made last week where he was asked about Duke's endoresement by a reporter. The clip shows him saying, "I didn't even know he endorsed me. David Duke endorsed me? Alright, I disavow it, OK?"

Trump hasn't always claimed ignorance on Duke's history. In 2000, he wrote a New York Times op-ed explaining why he abandoned the possibility of running for president on the Reform Party ticket. He wrote of an "underside" and "fringe element" of the party, concluding, "I leave the Reform Party to David Duke, Pat Buchanan and Lenora Fulani. That is not company I wish to keep."