By suggesting criminal charges could be in the cards for President Trump, the House committee investigating the events of January 6 isn’t just weakening their case before the Supreme Court. They’re also skating close to the constitutional prohibition on bills of attainder. That’s one of the limits Americans placed on Congress, enjoining what the Supreme Court has called “trial by legislature” — exactly what the January 6 committee appears eager to conduct.

The Constitution forbids bills of attainder as “a general safeguard against legislative exercise of the judicial function,” Chief Justice Warren explained in United States v. Brown. Under the British monarchy, such bills were used to punish without a trial individuals accused of disloyalty, treason, or any conduct the parliament found objectionable. The Framers, Chief Justice Warren wrote, “sought to guard against such dangers by limiting legislatures to the task of rule-making.”

Members of the January 6 committee seem to have lost sight of that wisdom. So far, they’ve been racking up victories in their effort, in federal court, to breach Mr. Trump’s executive privilege. The Supreme Court, though, has yet to decide whether to hear Mr. Trump’s appeal asking to shield his papers. It strikes us that the committee’s chairman has handed Mr. Trump’s lawyers valuable ammunition of Congress’s intent.

The misstep came but hours after Mr. Trump filed his appeal to the high court. “I can assure you that if a criminal referral would be warranted,” Congressman Bennie Thompson of Mississippi told the Washington Post, “there would be no reluctance on the part of this committee to do that.” On Sunday Congresswoman Liz Cheney said she saw “a number” of “potential criminal statutes at issue.” If the committee does dredge up evidence of illegality by Mr. Trump, it will be passed on to prosecutors.

Yet even the New York Times concedes criminal investigation is “not the primary mission of the committee,” noting today the “substantial political impact” a referral to the Justice department would have. Mr. Trump’s lawyers contend Mr. Thompson has “admitted his goal of uncovering information that could result in a criminal referral to the Department of Justice.” Mr. Thompson’s remarks prove, they say, that his committee “is acting as an inquisitorial tribunal seeking evidence of criminal activity.”

The committee, established by a resolution of the House, is mandated to “investigate the facts, circumstances, and causes” of the January 6 attack on the Capitol. “It cannot,” Mr. Trump’s lawyers maintain, “embark on what is essentially a law enforcement investigation, with the excuse that it might legislate based on information it turns up in the course of the exploration.” The committee needs a “valid legislative purpose.”

We’re not suggesting Mr. Trump will win on this head. We are suggesting it’s a serious point. It just might appeal to Chief Justice Roberts. He wrote last year, in Trump v. Mazars USA, that a congressional request for Mr. Trump’s tax records had raised “special concerns regarding the separation of powers.” He noted “Congress has no enumerated constitutional power to conduct investigations or issue subpoenas,” even if “each House has power ‘to secure needed information’ in order to legislate.”

Legislating seems to be the last thing on Mr. Thompson’s mind. He’s focused on what he deems Mr. Trump’s “delayed response to the Capitol attack,” as the Post describes it. “That dereliction of duty causes us real concern,” Mr. Thompson said, adding “one of those concerns” is “whether or not it was intentional.” He suggested that could be the basis for prosecution, which against a former president would be, the Post notes, “historic.”

Yes, but only insofar as it would be a constitutional gaffe of historic proportions. The Constitution already outlines a clear roadmap for the Congress in cases of alleged presidential crimes: impeachment. For Mr. Trump’s actions in respect of January 6, the House already accused Mr. Trump — and the Senate already acquitted him. So Congress is pursuing not only an attainder but a kind of double jeopardy.

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Drawing by Elliott Banfield, courtesy of the artist.