One thing to do while President Trump and the rest of the country wait for the Mueller Report to become public is to read a case known as Ex Parte Perry. That is, or was, the case in which Governor James Richard “Rick” Perry was brought up on charges by a prosecutor in Texas for threatening to veto the budget of the state’s Public Integrity Unit — while it was looking into his own administration.

What makes the case so apt just now is that it’s about whether the head of a government can be held criminally liable for using his constitutional powers to halt an investigation. That’s what the Democrats have been laying to President Trump. That’s what the state of Texas, or at least a left-wing prosecutor, laid to Governor Perry. It threw a spanner in Mr. Perry’s works, at least for a bit.

We don’t yet know what, if anything, Mr. Mueller might have said in his still secret report about all the chatter about obstruction by President Trump. There has been, though, plenty of loose talk, in the press and on the Hill, in respect of whether Mr. Trump obstructed justice in, say, the firing of the FBI’s director, James Comey. It turned the press into a political lynch mob.

What a contrast to the case that ended with Ex Parte Perry. The indictment of the governor was handed up in 2014. It clobbered him with a felony charge for threatening to veto $7.5 million for the Public Integrity Unit. It laid up another felony charge against Mr. Perry for seeking the resignation of a district attorney who’d been arrested for drunk driving and who also had oversight of the Public Integrity Unit.

The charges were widely recognized as constitutionally ridiculous (even the Times, the Washington Post, and other liberal outlets joined conservatives in defending Mr. Perry’s constitutional prerogatives). The lower courts in Texas, though, gave a mixed performance. Mr. Perry was finally cleared by the Lone Star’s highest court, which threw out all the charges.

Yes, the court said in a six-to-two decision, “prosecuting the exercise of a veto under the ‘abuse of official capacity’ statute is a violation of the Separation of Powers provision of the Texas Constitution.” Yes, it also said, the portion of the “coercion of a public servant” statute “used to prosecute the threat to exercise a veto” was “facially unconstitutional in violation of the First Amendment.”

Which brings us back to Mr. Trump. One would think that, by the time an analogous situation to Ex Parte Perry had arisen in the Trump presidency, the liberal camp would have been properly humbled. Instead, they started gigging Mr. Trump for using constitutional powers — in this case, to fire the head of the FBI with, or without, cause — as if Perry had never been handed down.

What, though, about Mr. Mueller? That is one of the things for which everyone is waiting. Will he suggest that Mr. Trump broke the law or regulation by firing Mr. Comey? Or will he defer to Mr. Trump’s constitutional powers to fire the head of the FBI, and other officers of the government? That power, the courts have long suggested, is part of the power to commission United States officers in the first place.