Some lies, however, may represent a dereliction of constitutional duty, and these cannot be left to the political marketplace.

As Philip Bobbitt shows in his supplement to Charles Black’s landmark study of impeachment, a “conspiracy to pervert the course of a presidential election” by “acting in league with a hostile foreign power” is clearly a basis for removal. So, too, is any false statement made to impede an investigation into this kind of conspiracy—including false statements to the public. A clear precedent is the article of impeachment on obstruction passed by the House Judiciary Committee in the proceedings against Richard Nixon. It included a charge that Nixon had made “false and misleading public statements,” which were “contrary to his trust as president and subversive of constitutional government.”

It is entirely possible that Trump has made “false and misleading public statements” of exactly that sort.

The president has repeatedly denied not only any legal offense of “collusion” with Russia leading up to the 2016 election—however he may understand the term—but also any contact at all between his campaign and the Kremlin. In November 2016, Hope Hicks spoke for him and his campaign in saying that there was “no communication between the campaign and any foreign entity during the campaign.”

Bob Bauer: Trump’s contempt for the law will be his downfall

But there was indeed such communication, including at Trump Tower on June 9, 2016, when high-level campaign officials met with a Kremlin-connected Russian lawyer and her associates offering “dirt” on Hillary Clinton. Although the president now takes the position that if there were contacts, he did not authorize them and knew nothing of them at the time, there are people who could falsify that claim.

Prominent among these witnesses is the former Trump campaign manager Paul Manafort, who is now cooperating with the special counsel in charge of the Russia investigation, Robert Mueller.

Contrary to the suggestion still periodically made in the president’s defense, collusion that consists of a campaign’s receipt of value from a foreign national, including but not only value in exchange for future policy concessions, would squarely present a violation of federal law.

But to put his onetime boss in jeopardy of impeachment, Manafort would not have to provide evidence of actual collusion—only that Trump knew of attempts to collude in one way or another, and repeatedly, publicly lied about them.

Manafort could, for example, shed light on what the president knew of the Trump Tower episode in advance, and when he learned about the results. On June 7, 2016, two days before the planned meeting, the president indicated that he would deliver a “major speech” about Hillary Clinton. Manafort might know if the president was getting ahead of himself, believing that Russians were on the way with “dirt” that Trump could use for such a speech. It has never seemed likely that if a foreign individual described as a “Russian government attorney” came to the United States for the declared purpose of sharing opposition research, the candidate would not have been informed. Manafort may have been the one to inform him, or would know who did.