Attorney General William P. Barr appeared before the Republican-led Senate Judiciary Committee last week to testify about his handling of special counsel Robert S. Mueller III’s report, but he refused to attend a similar hearing before the Democratic-run House Judiciary Committee the following day, citing lame concerns about the proposed hearing format.

Though Mr. Barr reiterated at the Senate hearing his view that he has no objection to Mr. Mueller testifying, the president undercut his attorney general’s position in a tweet, insisting that the special counsel should not appear before Congress.

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Then, on Monday, Treasury Secretary Steven Mnuchin refused to disclose Mr. Trump’s tax returns despite a lawful Democratic request, arguing that the Democrats had no legitimate legislative purpose to ask for them. And on Tuesday, White House counsel Pat A. Cipollone instructed his predecessor, Donald McGahn, to refuse to hand over documents relating to the special counsel probe. This occurred as the Justice Department and the House Judiciary Committee sought a compromise that would avert a committee vote Wednesday to hold Mr. Barr in contempt of Congress for refusing to turn over the unredacted Mueller report.

To be clear: The president cannot legally prevent Mr. Mueller from testifying, particularly once the special counsel formally exits the Justice Department. Congressional Democrats have the authority to demand Mr. Trump’s tax returns, and they have provided ample justification in the eyes of the law. And, though the unredacted Mueller report would seem to offer little more than what has already been released, the handling of Kenneth W. Starr’s investigation, in which Congress obtained voluminous amounts of information, suggests the Democrats are due more information in this case, too.

More to the point: Americans last year elected a Democratic House in part to act as a check on a wayward president. Neither Mr. Barr nor Mr. Trump gets to decide when Congress’s investigation into Russia’s election-year interference or into the president’s unjustifiable behavior ends. It would be particularly premature to declare these inquiries over now, as Congress is just grappling with the results of a two-year special counsel investigation — and before Mr. Mueller has even had an opportunity to clear up the spin the attorney general has offered lawmakers and the public about the investigation’s findings. Meanwhile, Mr. Mnuchin’s insistence that Congress has no legitimate legislative purpose to demand Mr. Trump’s tax returns shows reckless disregard for black-letter law, all to help Mr. Trump evade the transparency to which every president in decades has submitted.

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Several of these disagreements seem destined for lengthy court battles. That won’t serve the public interest. Previous administrations have sought to accommodate congressional overseers. But Mr. Trump’s “just say no” approach may mean that effective oversight may not come for years down the road.