The trial is the president’s opportunity to present a full-throated defense. There are many documents that could shed additional light on the president’s actions and several witnesses who have yet to testify. If exculpatory, I would expect the White House to welcome the production of the documents and urge the participation of the witnesses. They include the acting chief of staff, Mick Mulvaney, who publicly acknowledged that aid to Ukraine was tied to the prospect of investigations; the former national security adviser John Bolton, who described the president’s advisers’ attempts to compel Ukraine to intervene in our domestic affairs as “a drug deal,” and the senior White House budget official, Michael Duffey, who — we learned over the weekend — quietly ordered the Pentagon to freeze aid to Ukraine roughly 90 minutes after Mr. Trump asked the Ukrainian president for a “favor” on July 25th.

But so far, the president has directed these and other witnesses not to cooperate, and key documents have been withheld. That should change when the Senate holds a trial. During the Clinton trial, Senate Republicans passed a second resolution to call specific witnesses — even though those witnesses had previously testified extensively. Today, Mr. Trump is blocking critical witnesses from testifying at all.

The Senate should reject such stonewalling. It should not be complicit in a cover-up. We deserve to have the full story.

Nor should the Senate be complicit in promoting fact-free distractions and distortions. The president and some of his defenders have embraced objectively false and misleading defenses in the face of the House’s substantial evidence, including promoting a baseless theory that Ukraine interfered in the 2016 election. Even the president’s own national security advisers dismissed this theory as “completely debunked” and “a fictional narrative” that was “propagated by Russian security services.” The longer we permit cynically derived conspiracy theories to be equated with actual facts — with the truth — the more perilous the state of our republic.

How the Senate handles the coming trial will shape both the presidency and our constitutional system of checks and balances for decades, long after Mr. Trump leaves office. Will the Senate allow the president to abuse his public office to pursue wholly personal gains? Will the Senate permit the coercion of foreign governments to interfere in our domestic elections? Will the Senate enable the unprecedented, wholesale disregard of lawful congressional subpoenas to cover up the truth? Will the Senate make this the new norm?

I would not suggest to any senator that his or her oath requires at this time a specific verdict. Whether allegations are proven at trial is an entirely separate matter. But I strongly believe that our oath requires that all senators behave impartially and support a fair trial, one that places the pursuit of truth above fealty to this or any other president.

After vigorous debate, the framers included the power of impeachment in the Constitution for a reason. Presidents are not kings. Nor are they above the law. They can be removed for high crimes or misdemeanors, which means, according to Alexander Hamilton, an “abuse or violation of some public trust.”