A disturbing trend has emerged in the House impeachment inquiry of President Donald Trump: The White House is prohibiting key figures in the probe from cooperating, with Trump and his attorneys claiming sweeping executive immunity.

The latest example is former deputy national security adviser Charles Kupperman, who was subpoenaed to testify before Congress on Monday but refused to appear. Kupperman has filed a lawsuit, with his lawyer saying he will appear only if ordered to do so by a federal judge.

This is yet another attempt by Trump to escape accountability and reject the lawful oversight function of Congress as an equal branch of government. Trump’s defiance is rooted in his false notion that Congress somehow is subservient to him and that attempts to investigate possible wrongdoing constitute disloyalty to him personally and even treason.

The president’s blanket claim of executive privilege is so broad as to remove him from any type of scrutiny. To see how harmful his strategy is to the office of the presidency and the nation’s institutions, consider: His lawyers recently argued in court that Trump could not be charged with a crime while in office, that he literally is above the law.

That leaves the political and constitutional process of impeachment as the only other means of oversight. But here too, Trump wants to checkmate Congress, saying it cannot call the very witnesses who might attest to misdeeds. Further, he and his supporters are actively cultivating the idea that impeachment itself constitutes a “coup,” instead of the means by which a democratic society holds its leader to account.

Kupperman’s lawyer says his client feels caught between the demands of the White House and Congress. But Kupperman no longer works for Trump. Congress, on the other hand, can and should find him in contempt if he defies an order to appear.

Kupperman’s testimony is vital because he was among the handful of people who listened in on the July 25 call in which Trump infamously asked Ukrainian President Volodymyr Zelensky for “a favor” on investigating Joe Biden, with others attesting that a promised White House meeting and critical aid hung in the balance. Former national security adviser John Bolton has also been asked to testify. He was Kupperman’s boss and also in on the call.

Trump and his supporters have taken other means to ensure that the cost to individuals who defy him is high. Army Lt. Col. Alexander Vindman on Tuesday became the first person with firsthand knowledge of the call to testify. He corroborated earlier testimony by William Taylor, U.S. chief of mission for Ukraine, that inappropriate demands had been made of the Ukrainian government.

Vindman said he twice reported the incidents to national security officials, saying he was concerned they could harm national security. Within hours, Vindman — with a Purple Heart for combat duty in Iraq, a lifetime of service to his country, with a high clearance and an appointment by the Trump administration only last year — was being savagely disparaged in public by Trump and his supporters with little regard for his record of service.

That amounts to witness intimidation, with the clear message that anyone who dares to testify against this president will become a target. That’s a dangerous turn of events for this nation and its systems of checks and balances.