House Republicans, in their zeal to investigate Hunter Biden and his father, look poised to repeat the cardinal mistake of the January 6 committee. That error is breaching the constitutional prohibition of attainder, or “trial by legislature” as the Supreme Court calls it. “I want to be clear: This is an investigation of Joe Biden,” says Congressman James Comer, the expected head of the oversight committee, adding the probe will be the panel’s “focus.” 

We understand the concerns Mr. Comer and the GOP have as to “Joe Biden’s relationship with his family’s foreign partners,” and whether he “is compromised or swayed by foreign dollars and influence.” We caution, though, that this is the same path taken by the Democrats on the January 6 committee — a legislative body — when they violated the separation of powers by assuming a prosecutorial role against President Trump.

The January 6 committee’s disregard for the constitutional ban on attainder snapped into focus nearly a year ago as it was disclosed that it had hired 14 ex-federal prosecutors who were, the Times reported, “employing aggressive tactics typically used against mobsters and terrorists.” Its chairman even began muttering about using its findings to make a “criminal referral” — a proposed indictment of Mr. Trump — to the Justice Department.

The committee also showed its stripes in its effort to breach Mr. Trump’s executive privilege — a bedrock of the constitutional presidency dating back to George Washington’s refusal to hand over to Congress papers related to deliberations in respect of the Jay Treaty. Washington said the demand for his papers was not “relative” to “any purpose under the cognizance of the House of Representatives, except that of an impeachment.”

Such overstepping is why the Framers barred bills of attainder. Chief Justice Warren called it “a general safeguard against legislative exercise of the judicial function.” In Britain, attainder had been misused by Parliament to declare individuals guilty of such crimes as disloyalty, say, or treason. The Constitution “sought to guard against such dangers by limiting legislatures to the task of rule-making,” Warren observed.

Chief Justice Roberts underscored this principle in 2020 when he noted “significant separation of powers concerns” in a dispute that arose between Mr. Trump and the Congress over his tax records. The Chief Justice of America reminded that “Congress has no enumerated constitutional power to conduct investigations or issue subpoenas,” even though “each House has power ‘to secure needed information’ in order to legislate.”

Investigative demands by Congress, Chief Justice Roberts wrote, need to relate to making laws — “a valid legislative purpose,” as he put it. From inception, the January 6 committee strayed from this aim. Its enabling resolution asked it to “investigate the facts, circumstances, and causes” of January 6. Any legislative purpose was an afterthought, confined to the possibility of suggesting “corrective measures” in its final report. 

By taking on the role of prosecuting tribunal, the solons of the January 6 committee lost sight of their proper role as writers of laws and, instead, imagined themselves enforcers of it. For a historical analogue, one could look back to the House Un-American Activities Committee, another legislative panel with a righteous anti-communist cause that went after individuals and stained the reputation of the legislature.

This isn’t the first time your editors have crossed political lines over the way scandals are investigated in Washington. We opposed as unconstitutional, say, the use of an independent counsel against President Clinton, as well as against President George H.W. Bush. Now we’re opposing the use of attainder to go after a Democratic culprit for the same reason we opposed the use of attainder to go after President Trump.

Aside from the problem of separated powers, bigger and more urgent tasks await the 118th House than Mr. Biden’s crooked dealings. We need to rebuild our military and stock up on the kind of ammo such as we’re running through on behalf of a Free Ukraine. We need to deregulate our economy, and halt inflation. Most of all, begin the work of monetary reform the better to avoid a recurrence of the crisis the new House will inherit.