RUTHFULLY YOURS

Since the moment Mrs. Pelosi retook the gavel in January 2019, she’s operated as if her institution is the only branch in town, with limitless power. Within days of the 2018 election, an unnamed House Democrat bragged that the incoming majority was loading a “subpoena cannon,” aimed at more than 80 different areas of Trump investigation. The president’s tax returns, his firing of former FBI Director James Comey, his discussions with foreign leaders, security clearances, the Trump family business, Stormy Daniels, the reassignment of executive branch employees, the Mueller report. Ad nauseam.

No one disputes the House has oversight authority. But courts have always made clear that this power must be firmly tethered to a “legislative purpose.” Prior Congresses at least attempted to hew to the spirit of that phrase. Mrs. Pelosi’s committee chairmen—driven by fury over the Trump presidency—embarked on an extended fishing expedition. In doing so, they exposed how easy it is to abuse the legislative-purpose doctrine.

Case in point: Ways and Means Chairman Richard Neal in the spring of 2019 demanded six years’ worth of Mr. Trump’s tax returns, from 2013 to 2018. Mr. Neal knew he needed a legislative purpose, so he claimed his committee was conducting oversight of the Internal Revenue Service’s policy on auditing presidents. In case the pretext wasn’t already painfully obvious, he didn’t bother to ask the IRS about its audit policy, he didn’t ask for information about any other president, and he asked for Trump tax returns from four years before he was president.

The Democratic House also tossed overboard an established system of compromise. Congress and White Houses have long fought over documents and generally worked it out. Republicans as recently as 2017-18 spent more than a year using threats to pry documents out of the Justice Department about its Trump-Russia investigation and finally reached an accommodation. The Pelosi Democrats cared more about generating “obstruction” headlines than getting results. They refused to negotiate; they moved instantly to hold officials in contempt and file lawsuits.

These antics forced the Supreme Court to weigh in—and today’s Democrats, as well as future Congresses, may be sorry. In Trump v. Mazars, the justices blocked for now several congressional subpoenas for the president’s financial records, instructing lower courts to re-evaluate the issues. Mrs. Pelosi put a brave spin on this, claiming the justices had “reaffirmed Congress’s authority to conduct oversight.” In fact, the decision puts heightened scrutiny and new limits on Congress’s subpoena power.

Writing for a 7-2 majority, Chief Justice John Roberts took note of the House’s unrestrained behavior, explaining this dispute was “the first of its kind” to reach the high court after “two centuries” in which Congress and the White House were able to work such disagreements out. Yes, the court held, Congress can continue pushing for its subpoenas in lower courts, but henceforth all courts will be required to subject all subpoenas to several tests.

For starters, lawmakers will have to show that they need the president’s papers specifically to fulfill a legislative purpose. “Congress may not rely on the President’s information if other sources could reasonably provide Congress the information it needs,” Chief Justice Roberts writes. Courts must also now insist on a subpoena “no broader than reasonably necessary to support Congress’s legislative objective.” Those subpoenas must provide “detailed and substantial” evidence of legislative purpose. And finally, courts must from now on specifically assess “the burdens imposed on a President by a subpoena,” because they come from a “rival political branch,” which could use them “for institutional advantage.”

That’s a lot of new hurdles to jump, and the immediate consequence is that the Pelosi subpoena cannon has been replaced with a rifle. House Democrats are going to have to work much harder to pry information from this administration. Should Democrats go to the courts, their subpoenas will be subjected to new scrutiny. Should they attempt to return to a tradition of negotiation, the administration may now prove less willing to play ball, knowing that it has some shot of victory in the judiciary.

And that may prove the dynamic going forward—for all future Congresses and White Houses. Will the prior spirit of compromise and negotiation ever reign again? This is all new territory. It’s not the high court’s fault; Democrats’ anti-Trump mania (yet again) created a separation-of-powers circumstance that required judicial intervention. The ruling is instead another reminder: For all Democrats caterwaul about Mr. Trump breaking “institutions” and “norms,” it’s the “resistance” left that has produced the most wreckage.

Write to kim@wsj.com.