There is an important missing witness in the impeachment of President Trump, and his last name, for all the fulminations of the president’s defenders, isn’t Biden.

No, that witness is Trump himself — and the best case for calling him has been established by an argument advanced by the president’s own lawyers. Trump’s testimony is actually pertinent for precisely the reason the Bidens’ testimony is not.

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. If a president makes a reasoned decision about what best serves the nation’s interests, even if he turns out to be wrong, he has committed no impeachable offense. The Framers didn’t intend, through impeachment, to transform such policy disputes or mistakes into high crimes.

The claim that Trump acted for legitimate reasons ostensibly serves as the basis for his backers’ suggestions that former vice president Joe Biden or his son Hunter should be called to testify. The theory of summoning the Bidens would be to lend credence to the contention that Trump’s involvement in Ukraine was genuinely motivated by concern over corruption there.

But that argument presumes Trump has to meet a higher burden in showing his innocence than he actually bears. Precisely because the Constitution allows presidents to make non-corrupt mistakes in judgment, the true facts about Hunter Biden’s well-paid service on the board of a Ukrainian energy company or the vice president’s role in ousting a Ukrainian prosecutor don’t strictly matter. 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.

All of which means that it’s Trump, and not the Bidens, who should take the stand here. Trump needn’t come to the well of the Senate for that; in deference to the presidency, he could be allowed to testify from the White House, the venue from which President Bill Clinton testified before independent counsel Kenneth W. Starr’s grand jury two decades ago.

But if Trump were to testify, it wouldn’t be enough for him to baldly assert that his motives were pure, or “perfect,” as he is wont to say. And even if he truly believes he acted in good faith, that wouldn’t be enough to acquit him, either. The president’s duty to faithfully execute his office includes not only a duty of loyalty to the nation but also a duty of care — a duty to act with reasonable diligence and upon a reasonable basis. President George W. Bush, for example, couldn’t have been impeached merely because he blundered into a war in Iraq. But Bush could be impeached if he decided to launch the invasion based on the advice of a Ouija board or a Magic 8-Ball.

Which means that the cross-examination of witness Trump would determine his fate. Trump would have to answer specific questions about what he did, what he knew and when he knew it:

Your advisers told you that the Ukranians didn’t interfere with the 2016 election, but that the Russians did, right? You, yourself, ordered that the aid to Ukraine be held up? And your advisers repeatedly told you that was a bad thing, right? You were told about the whistleblower complaint before it became public, weren’t you? It was after you knew about that complaint that you told Ambassador Gordon Sondland that you didn’t want a quid pro quo, isn’t that true? And you didn’t release the aid until after you were told about the complaint, right? In fact, you released the aid only after the House announced it was investigating the whistleblower’s allegations, correct?

He’d be confronted on cross-examination with the incriminating statements he and others have made, which the House managers have already used compellingly in their opening arguments:

Here’s video of you on the South Lawn saying that what you wanted on the July 25 call was for Ukrainian President Volodymyr Zelensky to “start a major investigation of the Bidens” — you said that, right? You also said the Chinese should investigate the Bidens, too, didn’t you? And when you spoke to Sondland on July 26, you asked him whether Zelensky was going to do the investigations you wanted, didn’t you?

Confronted by a skilled examiner, Trump would melt down in minutes. 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’s wording, “opened the door” to calling Trump.

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