If he was simply serving in the role he prefers to serve—that of Donald Trump’s defense counsel—William Barr might be afforded a measure of leeway in his dramatic clash with the House Judiciary Committee. But Barr is the attorney general of the United States, and his arrogant refusal to cooperate with the committee will not end well.

Barr wants to set the rules of engagement in a way that would disempower the committee and deny the dictates of the Constitution. To that end, he is griping about a plan to have Judiciary Committee staff members—many of whom are experts in the areas that need to be addressed—join lawmakers in posing specific questions with regard to allegations that the attorney general mishandled and mischaracterized the report produced by special counsel Robert Mueller.

But the relationship with Congress that Barr demands is not the relationship that the Constitution demands.

Anyone who knows anything about the system of checks and balances knows that the Constitution is not on Barr’s side. It is on the side of Judiciary Committee chair Jerry Nadler as he pushes back against a compromised attorney general and the lawless president Barr serves.

“He is trying to blackmail the committee,” Nadler says of Barr’s refusal to appear. “We cannot allow the administration to dictate how we operate.”

This is not just a committee chairman declaiming on the latest outrage.

This is a serious student of the Constitution declaring that the legislative branch of our federal government has a duty to reassert itself as the essential overseer of an out-of-control executive branch. For too long, the Judiciary Committee has failed in this responsibility. Nadler recognizes that another failure at this point would have dire consequences for the future.

The chairman speaks from a place of deep knowledge and even deeper concern. In addition to his refusal to testify, Barr is ignoring a subpoena from the committee for the unredacted Mueller report and for the underlying evidence that was obtained as part of the special counsel’s inquiry into interference with the 2016 presidential election. The unredacted report and the documents associated with it are necessary tools for the Congress, which needs all the available information in order to get to the bottom of allegations of wrongdoing on the part of the president and his aides and allies.

“The challenge we face is bigger than a single witness,” explains Nadler, who noted Thursday that Barr’s Department of Justice has “made no meaningful attempt at accommodating that subpoena.

“The letter (from the Department of Justice) references the attorney general’s offer to twelve members of Congress—only twelve out of 535—to look behind some, but not all, of the redactions, provided that we agree not to discuss what we see with our colleagues and that we leave our notes behind at the Department of Justice,” says the chairman. “It is urgent that we see the documents we have subpoenaed. But I cannot agree to conditions that prevent me from discussing the full report with my colleagues, and that prevent the House from acting on the full report in any meaningful way. An accommodation designed to prevent us from taking official action is no accommodation at all.”

Nadler, who for many years served as the chair of the Judiciary Committee’s subcommittee on the Constitution, Civil Rights and Civil Liberties, is one the steadiest defenders of the Constitution in general and the system of checks and balances in particular.

He recognizes that what is at stake now is no small matter. This is a fight over the future of Congress.

Nadler has a sense of history, and he spoke to it well on Thursday, with a statement to the committee and to the country that fully embraced the oath he has sworn to “support and defend the Constitution of the United States against all enemies, foreign and domestic” and to “bear true faith and allegiance to the same.” He concluded with a bracing call for the Congress to assume its right and necessary stance:

Ladies and gentlemen, the challenge we face is that the President of the United States wants desperately to prevent Congress—a coequal branch of government—from providing any check whatsoever to even his most reckless decisions.

The challenge we face is that if we don’t stand up to him together, today, then we risk forever losing the power to stand up to any President in the future.

The Attorney General of the United States is sworn to uphold the Constitution as our nation’s chief law enforcement officer. He has an obligation to do everything in his power to warn the President of the damage he risks and the liability he assumes by directly threatening our system of checks and balances.

Sadly, the Attorney General has failed in that responsibility. He has failed to check the President’s worst instincts. He has not only misrepresented the findings of the Special Counsel. He has failed to protect the Special Counsel’s investigation from unfair political attacks. He has himself unfairly attacked the Special Counsel’s investigation. He has failed the men and women of the Department by placing the needs of the President over the fair administration of justice. He has even failed to show up today.

“Yes, we will continue to negotiate for access to the full report. And yes, we will have no choice but to move quickly to hold the Attorney General in contempt if he stalls or fails to negotiate in good faith. But the Attorney General must make a choice. Every one of us must make the same choice. That choice is now an obligation of our office. The choice is simple: we can stand up to this President in defense of the country and the Constitution we love, or we can let the moment pass us by.

I do not know what Attorney General Barr will choose. I do not know what my Republican colleagues will choose. But I am certain that there is no way forward for this country that does not include a reckoning with this clear and present danger to our constitutional order.

History will judge us for how we face this challenge. We will all be held accountable, one way or the other. And if he does not provide this Committee with the information it demands and the respect that it deserves, Mr. Barr’s moment of accountability will come soon enough.

This is a clash that could well escalate. “Yes, we will continue to negotiate for access to the full report,” says Nadler. “And yes, we will have no choice but to move quickly to hold the attorney general in contempt if he stalls or fails to negotiate in good faith.”

But this is not a clash between men. This is a clash between two understandings of how the United States will be governed. Barr would make the occupant of the Oval Office what the founders feared, an elected potentate serving as “a king for four years.” Nadler says, “We will make sure that no president becomes a monarch.”