Conservative radio host Hugh Hewitt expressed disbelief on Monday with Donald TrumpDonald John TrumpKoch-backed group launches six-figure ad buy against Heitkamp Anti-abortion Dem wins primary fight Lipinski holds slim lead in tough Illinois primary fight MORE’s handling of support from white nationalist David Duke.

“It’s inexplicable,” he told host Carol Costello on “CNN Newsroom." "I don’t know what Donald was thinking yesterday.”

Hewitt argued that Trump must have familiarity with Duke given the former Ku Klux Klansman’s notoriety.

“I can’t imagine anybody who wants to be president not knowing who David Duke is,” he said. "[Trump] had denounced David Duke two days ago.”

Hewitt also noted that Trump pleading ignorance on Duke’s background recalls some of the greatest presidential campaign gaffes in American history.

“It’s like Mitt Romney’s ’47 percent’ moment,” he said, citing the 2012 GOP presidential nominee’s charge that at least “47 percent” of people would reelect President Obama because of their dependency on big government.

“It’s like [former president] Gerald Ford saying ‘Poland is free’ during the 1976 debates,” Hewitt continued, referencing Ford’s remarks that the Soviet Union did not dominate Eastern Europe.

Hewitt added that Democratic presidential front-runner Hillary ClintonHillary Diane Rodham ClintonKoch-backed group launches six-figure ad buy against Heitkamp Trump keeps up 'low IQ' attack on Maxine Waters GOP leaders to Trump: Leave Mueller alone MORE has missteps of her own, but that Trump’s struggles with Duke’s backing would eclipse them.

“It’s like Hillary Clinton saying she left the White House ‘impoverished,” he said. "It’s like Hillary Clinton saying she was named for [famed explored] Sir Edmund Hillary. She’s got a long list of these as well but none of them would be as devastating on election weekend in November as this one.”

Trump on Monday blamed a “lousy earpiece” for confusing an exchange with CNN anchor Jake Tapper over Duke’s backing the day before.

Duke told listeners on his radio show last week that they should vote for Trump, arguing casting ballots for Sens. Marco RubioMarco Antonio RubioRubio: McCabe 'should've been allowed to finish through the weekend' For Tillerson, bucking Trump became a job-killer At least six dead after pedestrian bridge collapses on cars in Florida MORE (R-Fla.) and Ted CruzRafael (Ted) Edward CruzCruz says Cambridge Analytica assured him its practices were legal Dem battling Cruz in Texas: ‘I can understand how people think this is crazy’ Overnight Tech: Facebook faces crisis over Cambridge Analytica data | Lawmakers demand answers | What to watch for next | Day one of AT&T's merger trial | Self-driving Uber car kills pedestrian MORE (R-Texas) instead constitutes “treason to your heritage."

Trump has since repeatedly distanced himself from Duke’s praise, arguing he is unfamiliar with the full extent of his role in white nationalism.