The most riveting moment of the Supreme Court arguments today in respect of President Trump’s tax returns came from Justice Stephen Breyer. It was when the practical-minded, center-left justice hauled off and, so to speak, smote the lawyer for the House of Representatives with the jawbone of an ass … pardon … of Old Joe McCarthy. It was the bluntest warning of the kind of demagoguery the court could unleash upon the President.

The exchange came during the argument on the subpoenas issued by three House committees seeking years of Mr. Trump’s tax returns and other papers. The House’s lawyer, Douglas Letter, was trying to convince the court that the President himself wouldn’t be burdened. That’s when Justice Breyer wheeled on the House counsel to say that the subpoenas he’d seen certainly could impose a burden on the Presidency.

Such subpoenas, the Justice said, “apply to 15 Trump-affiliated entities. They ask for all documents related to opening of accounts, due diligence, closings, requests for information by other parties, etcetera. . . . If somebody subpoenaed you for that information or subpoenaed your tax accountant or subpoenaed somebody in your business, wouldn’t you at least want to know what was being turned over?”

“Wouldn’t you want to ask them? And might that take time? And might that not take effort? So my problem is there may be burdens here.” Justice Breyer said he wasn’t bothered that the House and Senate, like the President, are political. “The fact that what I hold today will also apply to a future Senator McCarthy asking a future Franklin Roosevelt or a future Harry Truman exactly the same questions, that bothers me.”

None of the subpoena recipients, the House counsel cooed, “have complained about burdens.”

“I’m sorry to interrupt you,” Justice Breyer interrupted, having had his fill of evasions. “I’m not talking about their burden. I’m talking about the President’s burden — in having to monitor, decide if there are privileges, figure out what his answers are to all those documents you are requesting which go, in my opinion, way, way, way beyond just tax returns.” Let us just say it was a memorable moment.

Whether Justice Breyer will decide the case is hard to guess. He also seemed perplexed as to why courts couldn’t weigh the presidential burdens in the ordinary course. The court more generally was hard to read, too, on whether it will permit the Manhattan DA, Cyrus Vance, Jr., to go after the President’s tax returns and other information being held by the accounting firm of Mazars. That, of course, is a state case.

Our own reading of the Constitution puts us in mind that the parchment favors the President on both sets of subpoenas — those from the House and those from New York County. We’d block Congress on, among other points, separated powers, enumerated powers, and the prohibition on bills of attainder, in which a legislature pursues an individual. We’d block Mr. Vance over federal supremacy and Thomas Jefferson’s warning.

The Virginian feared that permitting a president to be haled in state, or any, courts would result in him being dragged “from pillar to post.” We cited that warning when we defended, in President Clinton, a Democrat. We’ve cited it in defending, in Mr. Trump, a Republican. Joe McCarthy was nearly 200 years in the future when Jefferson issued his warning. It was nice to see Justice Breyer pick up the point.