The public impeachment hearings this week will be at least as important for what is not said as for what is. Congress will no doubt focus a lot on President Trump’s dealings with Ukraine and his secret plan to get that government to announce a public investigation of the man he considered his chief political rival, Joe Biden.

But think about what the president is trying to hide in the hearings. He has been blocking government officials from testifying before Congress, invoking specious claims of constitutional privilege. And while the Ukraine allegations have rightly captured the attention of Congress and much of the public, Mr. Trump’s effort to hinder the House investigation of him is at least as great a threat to the rule of law. It strikes at the heart of American democracy — and it is itself the essence of an impeachable offense.

President Trump has categorically refused to cooperate with the impeachment investigation. He has declined to turn over documents related to the inquiry and has instructed all members of his administration not to testify before Congress. Every member of the executive branch who has gone to tell the truth to the House impeachment investigators — like Marie Yovanovich and Alexander Vindman (and maybe Gordon Sondland, too, at least the second time around) — has done so in defiance of the president’s instructions. President Trump’s personal lawyer, Rudy Giuliani, has refused to testify. Secretary of Defense Mike Esper, Energy Secretary Rick Perry, Secretary of State Mike Pompeo and the acting White House chief of staff, Mick Mulvaney, have ignored congressional subpoenas related to the investigation.