Michael Cohen is sworn in to testify before the House Oversight Committee on Feb. 27. (Jim Watson/AFP/Getty Images)

IN THE showdown between House Democrats and President Trump, the White House seemed to have the high cards. Mr. Trump vows to defy “all the subpoenas” because congressional overseers “aren’t, like, impartial people” — true of every congressional investigation in history. This unprecedented, unjustifiable promise looked like a sure bet for a president indifferent to the public interest: If he never has to disclose documents or furnish witnesses, great; if courts eventually force disclosure, it may take so long he could be safely reelected first. Congress has limited means to enforce its urgent and legitimate interest in investigating the executive branch, putting the constitutional balance between executive and legislature alarmingly out of whack.

But now U.S. District Court Judge Amit P. Mehta has announced that he will fast-track his consideration of one of the key disputes between House Democrats and the president: the fate of a House Oversight Committee subpoena for Mr. Trump’s financial records at accounting firm Mazars USA. The committee wants the records to follow up on testimony from former Trump fixer Michael Cohen, who accused the president of knowingly manipulating his financial records in his business dealings. Rather than engage in a lengthy, multistep process, the judge declared Thursday that he could “discern no benefit from an additional round of legal arguments” nor “an obvious need to delay ruling on the merits.” Mr. Mehta will instead hold hearings Tuesday and make a final decision shortly thereafter.

After he rules, appellate judges should adopt Mr. Mehta’s sense of urgency. Waiting months for judges to settle a dispute is not normally seen as unreasonable. But these are not normal circumstances. Delay in this situation could become a de facto decision, confirming in effect that the president can stonewall congressional investigators and determining the effective power of the legislature relative to the executive.

Of course, all of this could be mooted if Mr. Trump changed his position on accommodating congressional requests and just did the right thing. Every president clashes with Congress; Mr. Trump’s congressional defenders not long ago were holding Eric H. Holder Jr., President Barack Obama’s attorney general, in contempt because he refused to disclose material on the “Fast and Furious” gunrunning scheme. But no president in recent memory has made blanket noncooperation administration policy the way Mr. Trump has. Whereas in the past many disputes were settled, if not amicably, at least without recourse to the courts, judges seem destined to rule in several serious cases in which Mr. Trump or his deputies have fought congressional demands.

When they do, they should emulate Mr. Mehta and move quickly. Mr. Trump should get a fair hearing in the courts, but he should not be allowed to run out the clock.