The prospect that President Clinton and Secretary of State Hillary Clinton could be prosecuted for failing to appear before Congress is being touted by Congressman James Comer of the House Oversight Committee. The Clintons have both ignored his subpoenas to testify about the disgraced financier Jeffrey Epstein. We have a hard time understanding why the Congress wants to hear from the Clintons on this head in the first place.

We carry no briefs for the Clintons. Yet it appears as if the erstwhile president and secretary of state have the better of the argument. They made their case this week in an extraordinary letter that accuses Mr. Comer and his allies of launching a process “literally designed to result in our imprisonment” by “trying to punish those you see as your enemies and to protect those you think are your friends.” They accuse their foes of a “dismantling of America.”

The Clintons’ best argument for avoiding the subpoenas is not panicky prose but constitutional bedrock. Congress’s power to subpoena is nowhere mentioned in the parchment. The Supreme Court has determined that a subpoena must be connected to a “legislative purpose.” Mr. Comer cites “oversight of the federal government’s enforcement of sex trafficking law generally and specifically” the investigation into Epstein and Ghislaine Maxwell.

Yet the high court has ruled that Congress mayn’t serve as a “law enforcement or trial agency.” That is marked in the Constitution’s prohibition on passing any bill of attainder or ex post facto law. That means that the legislature is prohibited from imposing punishment by legislative fiat or a trial in Congress. We’ve suggested from the start that the House January 6 committee ran afoul of this absolute, unembroidered plain-language.

Now Republicans are set to do the same. Mr. Comer insists that “No one’s accusing Bill Clinton of any wrongdoing. We just have questions.” That hardly cures the process, though — it merely steers it into a due process morass. The release of the so-called “Epstein Files” has inflicted damage on those who have never faced even the whisper of a criminal charge. The same objection goes for Special Counsel Jack Smith’s musings on President Trump’s guilt.         

The Clintons also argue that “any attempt to require testimony from a former president infringes the prerogatives of the Executive Branch.” That concern appeared to have also worried Alexander Hamilton. He predicted in 71 Federalist that the legislature would seek to exert an “imperious control” over the Executive Branch. The Supreme Court in Mazars v. Trump reckons that limitations on subpoenas are one way to resist that control.

The justices held in Mazars that there are “significant separation of powers issues raised by Congressional subpoenas for the president’s personal information.” That holds just as well for a Clinton as a Trump. We grasp that photographs have surfaced of Mr. Clinton in the company of Mr. Epstein and his entourage, and that Ms. Maxwell attended Chelsea Clinton’s wedding. A criminal contempt charge, though, requires far more than prurience.

So why is a conservative newspaper springing to the Clintons’ defense? It is that we are, as often noted, less interested in persons than in principles. One of the reasons that the Constitution is needed in the first place is to mark these principles. We supported Mr. Trump when the J6 Committee set its sights on him, and came out against the release of Mr. Smith’s final report. Due process was at stake in both cases. As it is here.