George Conway George Thomas ConwayGeorge and Kellyanne Conway honor Ginsburg Lincoln Project releases new ad blasting Trump as 'a horrible role model' George Conway hits Trump on 9/11 anniversary: 'The greatest threat to the safety and security of Americans' MORE, a conservative lawyer and husband to White House adviser Kellyanne Conway Kellyanne Elizabeth ConwayGeorge and Kellyanne Conway honor Ginsburg Trump carries on with rally, unaware of Ginsburg's death George Conway hits Trump on 9/11 anniversary: 'The greatest threat to the safety and security of Americans' MORE, said the key witness missing from the impeachment trial was President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE.

Conway, a frequent and often vociferous critic of Trump, wrote in an op-ed for The Washington Post on Saturday that Trump, not Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE or Hunter Biden, should be called as a witness in the Senate impeachment trial. Conway also claimed the president would "melt down" if he was questioned by a "skilled examiner."

As as the Senate impeachment proceedings got underway this week, the central question has been whether new witnesses, such as former national security adviser John Bolton John BoltonDiplomacy with China is good for America The Hill's Morning Report - Sponsored by The Air Line Pilots Association - Pence lauds Harris as 'experienced debater'; Trump, Biden diverge over debate prep DOJ launches probe into Bolton book for possible classified information disclosures MORE, will be subpoenaed.

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Republicans are widely opposed to bringing in witnesses, and the White House asked several former and current members of the administration not to testify under subpoena in front of the House committees when the impeachment inquiry was underway late last year.

In the op-ed, Conway said the best case for calling Trump to the stand has been made by "the argument advanced by Trump's own lawyers."

"Trump’s lawyers contend that a president should not be impeached and removed for making a bona fide policy judgment, whether or not that judgment turns out to be misguided or wrong. On that point, they’re absolutely right," he wrote.

However, Conway said Trump should be called to testify to prove that he acted in good faith when making that policy judgement with regard to his dealings with Ukraine and the country's president.

"What does matter is Trump’s state of mind: What he actually believed, and what basis he personally had for that belief, should determine whether he committed an abuse of power," he added.

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Conway wrote theoretical, straightforward questions that Trump could be asked during his testimony and explained why his answers could be incriminating. Conway said that if Trump testified, he’d be “humiliated.”

“Confronted by a skilled examiner, Trump would melt down in minutes,” Conway wrote. “He’d be humiliated, and he knows it — which is why he’s too terrified to give testimony under oath, and why it won’t happen. But it’s the logical conclusion of the argument the president’s lawyers have been making. They have, to use Trump lawyer Jay Sekulow Jay Alan SekulowNow, we need the election monitors Judge denies Trump's request for a stay on subpoena for tax records Judge throws out Trump effort to block subpoena for tax returns MORE’s wording, 'opened the door' to calling Trump.”