Some day years from now, we predict, historians are going to look back on the work of the House select committee investigating the events of January 6 and recoil in horror. This will be because of the way the Congress has careered past the constitutional prohibition on attainder and plunged into an investigation of whether individuals committed crimes. It has, in our opinion, gone way past the constitutional red lines.

A report on this head is in the latest number of the Times. It doesn’t address the constitutional question per se. It does, though, lay out how the committee is “borrowing techniques from federal prosecutions, employing aggressive tactics typically used against mobsters and terrorists as it seeks to break through stonewalling from former President Donald J. Trump and his allies and develop evidence that could prompt a criminal case.”

It’s not just the constitutional prohibition on Congress passing bills of attainder, which are laws convicting individuals of a crime and are prohibited in the Constitution with the same bluntness with which the parchment prohibits, say, titles of nobility. To those who say the January 6 committee is not going to emit any “bill” to the House or Senate, we say that the investigation of an individual for a crime is itself an abuse.

Nor is the abuse a breach merely of the attainder prohibition. It’s also of separated powers. The Times reporters — Michael Schmidt and Luke Broadwater — note that the January 6 committee has “no authority to pursue criminal charges.” Yet “the committee is using what powers it has in expansive ways in hopes of pressuring Attorney General Merrick B. Garland to use the Justice Department to investigate and prosecute them.”

What was the point of separating powers to begin with other than to stop this kind of abuse? Separation is the most basic curb on the government’s powers. Yet, the Times reports, Congress’s January 6 investigation is “being run by a former U.S. attorney, and the top investigator brought in to focus on Mr. Trump’s inner circle is also a former U.S. attorney. The panel has hired more than a dozen other former federal prosecutors.”

Those are ex-Article 2 gumshoes wielding Article 2 powers for an Article 1 committee. They have browbeaten more than, the Times reports, 475 witnesses and issued more than 100 subpoenas, “including broad ones to banks as well as telecommunications and social media companies.” They’ve “swept up the personal data of Trump family members and allies, local politicians and at least one member of Congress.”

It’s clear, from the Times’s dispatch, that even some Democrats are nervous about the abuses. Messrs. Schmidt and Broadwater quote a former top lawyer for the House, Stanley Brand, now representing an aide to Mr. Trump, as saying that this is “a quantum leap for Congress in a way I’ve never seen before.” Says Mr. Brand: “They think they’re fighting for the survival of the democracy and the ends justify the means.”

“Just wait if the Republicans take over,” Mr. Brand adds. Messrs. Schmidt and Broadwater themselves warn of “another obvious risk” — that the committee could fail “to make a persuasive case for a Justice Department prosecution.” They note that Mr. Trump “survived years of scrutiny by the special counsel in the Russia investigation” and “two impeachments” and “remains the dominant force in Republican politics.”

It’s not our purpose here to belittle the notion that President Trump is a fit subjection for investigation. It is our purpose, as it has been throughout, to mark the importance of due process. The Times quotes Congressman Adam Schiff as insisting, “It’s not a criminal investigation.” That is not a credible statement. The House is violating — in broad daylight — a basic protection the Constitution accords to an individual.

Plus, too, it might turn out that the House is poisoning any proper prosecution that might follow. Whatever future historians might conclude, though, they will no doubt consult 44 Federalist, in which Madison lists attainder bills as among the abuses that are “contrary to the first principles of the social compact” and are prohibited by the Constitution as a “bulwark in favor of personal security and private rights.”