U.S. Sen. John Cornyn — a former state district judge, Texas Supreme Court justice and Texas attorney general — thinks it would be a great idea to open the FBI’s investigative files on Hillary Clinton.

“After Director [James] Comey dismantled every bit of Secretary Clinton’s explanation about her private email server that she’s been telling the American people over the last couple of years, I think the most important thing the Department of Justice could do, the most important thing the FBI could do, and the most important thing that she could do would be to make sure that all of the investigation by the Department of Justice and the FBI be made public,” he said, via news release Wednesday.

Wait. Isn’t there a reason for separating police and prosecutors and courts — and for separating all of them from politics? It’s so the police — this time, the FBI — can do their work without political interference. It is also supposed to protect candidates and officeholders from the criminalization of politics.

Cornyn understands that as well as anyone, or he should: Look at his resume. Yes, it is his job now, as the No. 2 Republican in the Senate, to make an even bigger political mess for the Democratic presidential nominee — a short-term gain for a GOP trying to make lemonade out of its own presidential lemon. But he would have to punch a big hole in the wall between politics and policing to be successful.

Consider that through the eyes of a non-political person accused of a crime. After an investigation, no charges are brought. And yet, the public gets to scour that insufficient evidence for hints of scandal and whiffs of gossip. The exoneration becomes a defamation.

Consider Cornyn’s idea as applied to the case of former Texas Gov. Rick Perry. Perry was charged with misusing his office by demanding the resignation of Travis County District Attorney Rosemary Lehmberg and threatening state funding for her office if she refused. She refused. He vetoed the funding. A special prosecutor appointed by a visiting judge — they wanted to squeeze as much “Liberal Austin” out of this as possible, see — took it to a grand jury, which indicted the governor. The charges were dropped after Perry challenged them in court, and he never went to trial.

By Cornyn’s reckoning, the files ought to be public.

But that’s nuts, even if it would give a political party — Republicans in the first case, Democrats in the second — something to talk about in an election year.

This is not a perfect world, but if it were, we would never even hear about investigations that didn’t produce charges and convictions. Investigators and grand juries are leaky, even though they’re supposed to be secret. Some prosecuting and defense lawyers and judges blab, if not to reporters directly, then to their families and friends, who then talk to their families and friends, and so on.

By Cornyn’s reckoning, the files ought to be public. But that’s nuts, even if it would give a political party — Republicans in the first case, Democrats in the second — something to talk about in an election year.

It’s a big, slimy mess, even if you don’t season it with politics. Politics makes it even slimier.

What law enforcement calls an investigation is really just a big question — what happened? — surrounded by a lot of related questions. It’s an inquiry, not an answer.

In politics, however, it’s an answer. If the FBI is looking under your hood, there must be a gremlin of some kind in there. Even if there’s not, we voters are gullible enough to go for it almost every time.

This is no apologia for Hillary Clinton and her “extremely careless” handling of the national secrets with which she was entrusted as Secretary of State. That’s in the hands of voters now.

Perhaps you can argue, as Cornyn seems to be doing, that there is more at stake in the Clinton case and that an exception ought to be made. If it is made, that exception will be the lever that opens the next case file and the next one after that.

It’s not even necessary. In this year’s presidential race, it’s hard to imagine something so terrible about one candidate that the other candidate’s supporters won’t repeat it, files or no files. Don’t worry. People will talk about Clinton and her emails and servers all the way to November.