Sen. Bernie Sanders Bernie SandersButtigieg stands in as Pence for Harris's debate practice Bernie Sanders warns of 'nightmare scenario' if Trump refuses election results Harris joins women's voter mobilization event also featuring Pelosi, Gloria Steinem, Jane Fonda MORE (I-Vt.) joked that he would “bring a lie detector” if he faces President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE on the debate stage.

Sanders, who announced this month that he is running for president, made the joke during a CNN town hall Monday night after host Wolf Blitzer asked how he planned to interact with Trump during a possible presidential debate.

“We’ll bring a lie detector along," Sanders said, prompting laughs from the audience. “And every time he lies it goes ‘beep.’ That would be the first thing.”

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After the quip, Sanders took a more serious tone, saying he plans to hold Trump “accountable” for his promises and actions during his presidency.

“I think the fraud that Trump is, the pathological liar that he is, has to be exposed,” the senator said. “We are going to hold him accountable for what he said and what he did.”

Sanders and other Trump critics have frequently pointed to the president’s falsehoods and misstatements in their criticism of him.

The Washington Post fact-checker has documented thousands of false claims during the first two years of Trump’s presidencies, and in December introduced the “Bottomless Pinocchio” rating to call out Trump and other politicians on repeated false claims.