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.

NOTE: "No one prevents a witness who can exonerate them from testifying." - former U.S. Attorney Joyce White Vance

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

Mr. Trump’s stonewalling is a grave problem because it means there is no way to police executive branch wrongdoing.

Attorney General William Barr has said a sitting president cannot be indicted. The president’s lawyers have gone so far as to say, in light of that principle, that he cannot even be criminally investigated.

But every serious scholar who adheres to the view that a sitting president cannot be indicted combines that view with the belief that the impeachment process is the way to deal with a lawless president.

Indeed, the very Justice Department opinions that Mr. Barr relied on to “clear” the president say exactly that. Otherwise a president could engage in extreme wrongdoing, and the American people would have no remedy.

How could a system that allows the subject of an investigation to block all the witnesses from testifying be consistent with the rule of law?

Just think about President Richard Nixon. He, too, tried to block White House officials from testifying in Congress.

“Under the doctrine of separation of powers,” Nixon declared on March 12, 1973, “the manner in which the president personally exercises his assigned executive powers is not subject to questioning by another branch of government.”

But the Senate Select Watergate Committee held firm and insisted on the witnesses appearing, going so far as to say it would jail any witness who invoked executive privilege.

That led Nixon to throw in the towel, saying he would not invoke privilege and would let the aides testify.

Trump's stonewalling is particularly pernicious here because Mr. Trump’s party controls the Senate. Why is the president afraid of letting his own White House officials tell the truth in a process ultimately controlled by Senate Republicans?

The bottom line is that President Trump is out-Nixoning Nixon. The American people deserve answers.

Any claim by the president to hide the truth is itself a grave wrong and an impeachable offense.