Unlike Nixon, Mr. Trump has now actually been impeached, for abuse of power and obstructing congressional investigation. If official witnesses don’t testify about these acts, the very subordinates who may have helped Mr. Trump commit them can aid and abet his continuing obstruction. If so, on what conceivable basis can such officials as Mr. Pompeo and Mr. Mulvaney continue to hold high office under an oath to support and defend the Constitution? And recent history only repeats itself if former officials can enrich themselves through memoirs based on what they learned in public office about Mr. Trump’s abuse of a public position for private gain.

Until now, public pressure to secure this testimony has been directed more at swing senators than at the recalcitrant witnesses. But these witnesses withhold their testimony in contempt of Congress and the law. Their actions create a new constitutional crisis, to which Congress must respond with four steps.