Conservative radio host Hugh Hewitt expressed disbelief on Monday with Donald TrumpDonald John TrumpJustice Department preparing for Mueller report as soon as next week: reports Smollett lawyers declare 'Empire' star innocent Pelosi asks members to support resolution against emergency declaration 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.”

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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 ClintonFBI’s top lawyer believed Hillary Clinton should face charges, but was talked out of it Harris adds key Clinton aide, women of color to 2020 campaign: report Democrats more likely Trump's foil, than to foil Trump 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 RubioThe Hill's 12:30 Report: Trump escalates fight with NY Times The Hill's Morning Report - Can Bernie recapture 2016 magic? Venezuela puts spotlight on Rubio MORE (R-Fla.) and Ted CruzRafael (Ted) Edward CruzO’Rourke not ruling out being vice presidential candidate CNN ripped for hiring former Republican operative as political editor: 'WTF?!?!' The Hill's Morning Report - Can Bernie recapture 2016 magic? 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.