Stephen Colbert says Donald Trump just failed the easiest test in American politics. The host of CBS’ “The Late Show with Stephen Colbert” mocked the Republican U.S. presidential candidate Monday over his refusal to disavow the public support of former Ku Klux Klan leader and white supremacist David Duke.

On CNN’s “State of the Union” Sunday, host Jake Tapper asked Trump whether he would “unequivocally condemn David Duke and say you don’t want his support.” Trump replied by sidestepping the question, claiming he did not know David Duke.

After lamenting the fact that Trump could effectively secure the GOP nomination in the Super Tuesday contests, Colbert chided the candidate over the arguably hypocritical answer. 

“Trump needs to know [David Duke] before condemning [him] or the KKK. It’s not like they are Muslim or born in Mexico,” Colbert joked, referring to the candidate’s controversial positions on Syrian refugees and illegal immigration. “He needs to think about it.”

Colbert went on to say that Trump’s dodge of Tapper’s query constituted an incorrect response to “the easiest question to answer in American politics.” The comedian joked, “When someone says, ‘Do you disavow the Ku Klux Klan?,’ just say yes and start high-fiving your future cabinet.”

Colbert also mocked Trump for blaming the gaffe on what the billionaire businessman claimed was a faulty earpiece he was wearing during the taped segment. “He blames this one on the earpiece, but I think he should take some responsibility for the mouthpiece — and also the hairpiece,” Colbert said, referring to the candidate’s trademark hairdo.

Watch Colbert mock Trump on “The Late Show with Stephen Colbert” below:

Heading into the highly important Super Tuesday nominating contests, Trump is the clear front-runner in the Republican race. He has won contests in New Hampshire, South Carolina and Nevada, and he could secure enough delegates Tuesday to make his lead over rivals close to insurmountable. Nationally, the RealClearPolitics average of GOP presidential polling data has Trump leading with 35.6 percent support, well ahead of U.S. Sen. Ted Cruz of Texas, with 19.8 percent, and U.S. Sen. Marco Rubio of Florida, with 17.4 percent.

Establishment Republicans hope missteps, such as the controversy over Trump’s David Duke-related comments, will help unseat the front-runner in the race. However, it may be too late for any other candidate to catch him.