William Barr’s testimony before the Senate Judiciary Committee on Wednesday was a sad affair. The attorney general proved himself willing to bend the law, distort facts, dismiss a genuine foreign threat to American electoral democracy, and embrace conspiracy theories that impugn the integrity of veterans and current employees of his own department—all in defense of presidential behavior that, whether criminal or not, should be indefensible by any American patriot. He followed this up by refusing to show up for scheduled testimony in front of the House Judiciary Committee because of an apparent fear of facing a proper cross-examination by the committee’s staff attorneys.

Despite all this, at first glance, Barr’s personal views on the weight of the evidence in the Mueller report and his opinion about whether the president did—or even can—commit the crime of obstructing justice would now seem quite irrelevant. Precisely because special counsel Robert Mueller adhered to Department of Justice policy and did not recommend indicting a sitting president, the question of what to do about Russian electoral meddling and about Trump’s arguably obstructive conduct has passed out of the hands of the Justice Department and into the hands of Congress.

However, if one looks more closely at Barr’s behavior, it becomes clear that, not content with blunting the public effect of the Mueller report, he is carefully maneuvering to frustrate congressional action as well.

Congressional authority over the subjects addressed in the Mueller report flows from two constitutional sources. First, Congress has the general power to legislate on matters such as the integrity of elections, relations with foreign powers, and the honest and efficient operation of the executive branch. Second, it has the particular power to impeach the president, vice president, and other civil officers.

If Congress is to craft appropriate prophylactic measures against the newly apparent danger of covert foreign interference in U.S. elections, it needs to understand the nature and extent of the threat. Volume I of Mueller’s report, and the evidence on which it is based, is a trove of indispensable information on this point. Likewise, it is Congress—not the attorney general—that is charged with determining whether particular presidential conduct amounts to a high crime or misdemeanor meriting impeachment. Thus, Congress cannot fulfill its constitutional role without access to both the full report and the bulk of its supporting evidence. House Judiciary Chairman Jerry Nadler previously issued a subpoena with a deadline of this past Wednesday for all of those materials. That deadline has come and gone.

Ultimately, then, the most consequential development on the day Barr went before the Senate Judiciary Committee was not his testimony, but a letter from Assistant Attorney General Stephen Boyd categorically declaring that the Justice Department would not obey the House Judiciary Committee’s subpoena.

Rightly understood, Barr’s pugnacious public performance had two purposes. The first, and obvious, objective was to spin or minimize the ugly facts exposed by those portions of the Mueller report already in the public realm in ways favorable to Trump. The second, more subtle but potentially more important, purpose was to frame the Mueller investigation in a way that will make it easier to continue to refuse congressional demands for all the evidentiary material upon which the report rests.

This second purpose requires some explanation.

Congress has broad investigative power, but the scope of that power in any particular matter depends on its constitutional basis. One basis is the “oversight” power, which arises from the recognition that Congress’s constitutional authority to legislate necessarily implies a power to investigate and to compel production of information necessary to legislation. The power of investigation is so indispensible to legislating that the Supreme Court has called the two powers “co-extensive.”

Nonetheless, because congressional legislative authority is not unlimited, there will be occasions on which separation of powers principles limit congressional investigative authority. One tricky area is the criminal justice system. On the one hand, the Supreme Court has held that Congress may properly investigate the operations of the Justice Department, including whether the department and its officers properly exercised their prosecutorial authority in particular cases. On the other hand, the court has also held that Congress cannot inquire into purely private affairs unrelated to legislation, nor substitute its proceedings for criminal law enforcement that the Constitution assigns to the executive and judicial branches.

Congress has broad investigative power, but the scope of that power in any particular matter depends on its constitutional basis.

That said, none of these limitations on congressional oversight authority apply to impeachment, an express constitutional power conferred exclusively on Congress and unreviewable by any court. Particularly where the subject of an impeachment inquiry is the president, and the conduct at issue is alleged interference with criminal investigative processes, there is no plausible constitutional ground upon which any executive agency can decline to provide Congress information it deems relevant.

Indeed, the Justice Department policy against indicting sitting presidents explicitly recognizes the primacy of Congress over presidential misconduct. The Office of Legal Counsel memo promulgated in 2000—upon which Mueller evidently based his decision not to issue a traditional prosecutorial judgment in the presidential obstruction case—argued that the president’s place in the constitutional structure, and the impediments to a functioning executive branch posed by criminal charges against a president, support barring such charges. But the same memo concluded by observing that the constitutional remedy for a misbehaving president is impeachment and that: “[T]he Framers themselves specifically determined that the public interest in immediately removing a sitting President whose continuation in office poses a threat to the Nation’s welfare outweighs the public interest in avoiding the Executive burdens incident thereto.”

The peculiar form of the Mueller report can only be understood as an effort to reconcile a conflicting set of internal DOJ directives. On the one hand, Mueller’s charge as special counsel was to investigate Russian election interference, cooperation with such interference by the Trump campaign, and any matters arising incidental to that investigation. This necessarily meant he was to investigate the president. On the other hand, DOJ policy barred him from indicting Trump regardless of the evidence he uncovered, and decreed that the exclusive remedy for presidential misbehavior is impeachment.

It seems fairly plain that Mueller concluded (a) the evidence supported a criminal case for obstruction against the president; (b) in deference to DOJ policy, he should not expressly make that determination; and (c) the proper venue for judging the evidence he gathered was Congress. Indeed, it is difficult to read the report as anything other than a referral to the legislature—an invitation for Congress to take and judge Mueller’s evidence itself. So long as the report is understood that way, it becomes well-nigh impossible for the Justice Department to refuse Congress when it accepts the invitation and asks for Mueller’s evidence.

But Barr’s choice, to decide in Trump’s favor the question of presidential guilt that Mueller had left open, did more than merely get out front of the news cycle. Critically, both Barr’s original letter claiming to summarize Mueller’s principal conclusions before the report’s release and his Senate testimony dovetail with the Boyd letter denying the House access to the Mueller evidence. By narrowly framing Mueller’s investigation and the resulting report as merely another criminal case over which Barr as AG has the final say, Barr effectively rescinds or nullifies Mueller’s implied invitation to Congress to take over where he left off. This framing also strengthens the argument in the Boyd letter that congressional requests for criminal investigative materials exceed its oversight authority and violate a norm against political interference in the federal criminal process.

In the end, the House Judiciary Committee would probably win a subpoena fight grounded purely in their oversight powers, but it would be a complex, protracted, and nuanced battle. Nadler could dramatically strengthen the House’s legal position and effectively neutralize all the arguments in the Boyd letter by declaring plainly that the committee’s request is made pursuant to the House’s constitutional authority to investigate impeachable conduct by a president. There is no legally supportable ground to refuse an evidentiary request made by Congress on that basis, and it would add an urgency to the proceedings that would surely expedite them.

From Nadler’s perspective, the problem with this easy solution is political, not legal. He and House Speaker Nancy Pelosi are deeply reluctant to make the formal declaration that the House or any committee is conducting an impeachment investigation. Because once they do so, they’re locked into following the evidence wherever it leads—and as a factual and constitutional matter, it probably leads to the conclusion that Trump has engaged in conduct traditionally considered high crimes and misdemeanors. But the Democratic leadership doesn’t want to be shunted onto that track with no way off it given the near-certainty of acquittal in the Senate, whatever the strength of the evidence, and the risk of losing the war of public opinion and thus the 2020 election.

For House Democrats, it’s an exquisite and agonizing dilemma. Their indecision is evident in Nadler’s letter on Friday responding to Boyd. He delicately dances around the question, referring to Congress’ “constitutional, oversight, and legislative interest in investigating misconduct by the President and his associates,” but he will not even pronounce the deadly word “impeachment.”

The Democrats’ discomfort is so evident that one is tempted to speculate that Barr is consciously trying to force Nadler and the House to assume a legally correct, but politically hazardous posture.

Barr’s performance in his second go-round as attorney general should disappoint any friend of American constitutional government and an independent Department of Justice. As a partisan street fighter, though, he’s demonstrated a special flair for political jiujitsu.