What precedent is set if obstructing such an investigation is allowed to go unpunished and undeterred? It is hard enough for the executive branch to investigate a sitting president, who has the power to fire a special counsel (if needed, through the attorney general) and to thwart cooperation with an investigation by use of the clemency power. We saw Mr. Trump use his clemency power to do just that with, for example, his ally Roger Stone. He commuted Mr. Stone’s sentence, who was duly convicted by a jury but never spent a day in jail for crimes that a federal judge found were committed for the president. The same judge found that Paul Manafort, a former Trump campaign chairman, lied to us repeatedly, breaching his cooperation agreement. He, too, was surely holding out hope for a dangled pardon.

Mr. Trump can’t point to what the special counsel investigation did not find (e.g., “collusion”) when he obstructed that very investigation. The evidence against Mr. Trump includes the testimony of Don McGahn, Mr. Trump’s former White House counsel, who detailed how the president ordered the firing of the special counsel and how when that effort was reported in the press, Mr. Trump beseeched Mr. McGahn to deny publicly the truth and, for safe measure, memorialize that falsity in a written memorandum.