SUNDAY, CHAIRMAN JERRY NADLER (D-New York) of the House Judiciary Committee threatened to subpoena Attorney General William Barr if he refuses to testify this week about the Robert Mueller report.

But a subpoena is unlikely to elicit Barr’s cooperation. “We’re fighting all the subpoenas,” said the president of the United States.

In other words, there is to be no congressional oversight of this administration. There is to be no questioning the attorney general about the Mueller report and a Trump adviser about immigration policy, either.

There is to be no questioning a former White House security director about issuances of security clearances or anyone about presidential tax returns.

Such a blanket edict fits a dictator of a banana republic, not the president of a constitutional republic founded on separation of powers.

If Congress cannot question the people who are making policy or obtain critical documents, Congress cannot function as a coequal branch of government.

If Congress cannot get information about the executive branch, there is no longer any separation of powers as sanctified in the U.S. Constitution.

There is only one power: the power of the president to rule as he wishes; which is what Donald Trump has sought all along.

The only relevant question is how to stop this dictatorial move.

Presidents before Trump occasionally have argued that complying with a particular subpoena for a particular person or document would infringe upon confidential deliberations within the executive branch.

But no president before Trump has used executive privilege as a blanket refusal to cooperate.

“If Mr. Barr does not show up, we will have to use whatever means we can to enforce the subpoena,” said Nadler.

What could the House Judiciary Committee do? Hold Barr in contempt of Congress, under the inherent power of Congress to get the information it needs to carry out its constitutional duties? Congress cannot function without this power.

Under this power, the House can order its own sergeant-at-arms to arrest the offender, subject him to a trial before the full House and, if judged to be in contempt, jail that person until he appears before the House and brings whatever documentation the House has subpoenaed.

When President Richard Nixon tried to stop key aides from testifying in the Senate Watergate hearings in 1973, Sen. Sam Ervin, chairman of the Watergate select committee, threatened to jail anyone who refused to appear.

Congress hasn’t actually carried through on the threat since 1935, but it could.

Would America really be subject to the wild spectacle of the sergeant-at-arms of the House arresting an attorney general and possibly placing him in jail?

Probably not. Before that ever occurred, the Trump administration would take the matter to the Supreme Court on an expedited basis.

Sadly, there seems to be no other way to get Trump to move. Putting the onus on the Trump administration to get the issue to the court as soon as possible is the only way to force Trump into action and not let him simply run out the clock before the next election.

What would the court decide? With two Trump appointees now filling nine of the seats, it’s hardly a certainty.

But in a case that grew out of the Teapot Dome scandal in the 1920s, the court held that the investigative power of Congress is at its peak when lawmakers look into fraud or maladministration in another government department.

Decades later, when Nixon tried to block the release of incriminating recordings of his discussions with aides, the Supreme Court decided that a claim of executive privilege did not protect information relevant to the investigation of potential crimes.

Trump’s contempt for the inherent power of Congress cannot stand. It is the most dictatorial move he has initiated since becoming president.

Congress has a constitutional duty to respond forcefully, using its own inherent power of contempt.

Robert Reich’s latest book is “The Common Good” and his newest documentary is “Saving Capitalism.”