Be careful for what one wishes. That’s our advice in the wake of a ruling by a federal judge in the Columbia district to strip President Trump of the immunity from civil litigation that the Constitution accorded America’s presidents. This January 6-inspired attempt to undermine the doctrine of separated powers — and the idea of the presidency itself — might come to haunt Democrats.

Friday’s opinion, by District Judge Amit Mehta, says Mr. Trump faces potential liability for the events of January 6, including the breach of the Capitol by protesters. The lawsuits, filed by House members as well as two Capitol Hill police officers, amount to an effort to punish Mr. Trump, the New York Times reported, “amid mounting frustration that he has faced no penalty for the riot.”

“Above all else, it’s about accountability,” a lawyer behind the suits, Joseph Sellers, told the Times. We carry no brief for Mr. Trump. What we are a partisan of is the presidency. The Congress, though, seems blithe to the fact that it already tried to bring President Trump to book for January 6 via a full-fledged impeachment. And it failed to convince the impeachment jury, who delivered a verdict of not guilty.

Mr. Trump’s lawyer, Jesse Binnall, says the civil suits to which the Congress is pulling out of its hat are “chock-full of propaganda” and “meant to achieve a political rather than a legal objective.” He argues that Mr. Trump’s actions on January 6 were protected as free speech by the First Amendment and, more importantly, by the Supreme Court’s long-standing precedent of “absolute immunity” for presidential actions.

Judge Mehta acknowledges the constitutional questions at stake. “To deny a President immunity from civil damages is no small step,” he purports to recognize. He then tosses the issue aside, notwithstanding a precedent, Nixon v. Fitzgerald, handed down as recently as 1982. That’s when the Supreme Court concluded that in respect of liability in civil suits the “Presidential immunity” is “absolute.”

This immunity applies, the Supreme Court found, only to acts that fall within what it called the “outer perimeter” of the President’s “official responsibility.” Yet the court was hesitant to etch that perimeter precisely, noting the president “has discretionary responsibilities in a broad variety of areas,” and “it would be difficult to determine which of the President’s innumerable ‘functions’ encompassed a particular action.”

Difficult indeed, though Judge Mehta finds no such difficulty. The President’ action on January 6 “do not relate to his duties of faithfully executing the laws,” the judge contends. On what basis does he wave aside Mr. Trump’s contention that his remarks and actions on January 6 were grounded in an effort to “take Care that the Laws be faithfully executed”? What could be more of a core presidential function?

Particularly if the Supreme Court has insisted that any president’s absolute immunity falls within the “outer perimeter” of presidential duties. We understand that the question could reach a Supreme Court that has not proven especially receptive to Mr. Trump’s arguments relating to January 6. Scotus has already refused to credit Mr. Trump’s privilege over his private communications on January 6.

All we can say is prenez garde. We still remember how all the Democratic do-gooders were all for using an “independent” counsel against President Reagan, only to find that it was a Democrat, President Clinton, who became its final victim. When it came time to renew the blasted independent counsel law, the Congress — Democrats and Republicans — sat on their hands, and the measure expired.

In Nixon v. Fitzgerald, the Supreme Court ordained that the president’s “absolute immunity” still left “sufficient protection against misconduct” by the President — “the constitutional remedy of impeachment.” The Democrats have unsheathed that sword twice against Mr. Trump. They got nowhere, but better to try a third time than to endanger future presidents by breaching the “outer perimeter” of the Constitution.