Donald Trump says he wants a special prosecutor to investigate Hillary Clinton if he becomes President, and our friends on the left are up in arms. “Banana republic” stuff, they cry. We agree, but where were they when Barack Obama did the same in 2008?

“If I win, I am going to instruct my Attorney General to get a special prosecutor to look into your situation,” Mr. Trump told Mrs. Clinton at Sunday’s debate. This is a mistake on several levels, not least because promising to prosecute political opponents if you win is essentially a promise to politicize the Justice Department. It’s what dictatorships or unhealthy democracies like Argentina do, and it breeds lack of trust and public cynicism. It might cheer Mr. Trump’s supporters, but we doubt it will reassure undecided voters about his presidential temperament.

Then again, where could Mr. Trump have conjured such a bad idea? Well, maybe from a certain Senator who ran for President in 2008 promising an investigation of the Bush Administration’s “torture” of jihadist detainees. Here’s how he put it in April 2008:

“What I would want to do is to have my Justice Department and my Attorney General immediately review the information that’s already there and to find out are there inquiries that need to be pursued. I can’t prejudge that because we don’t have access to all the material right now. I think that you are right, if crimes have been committed, they should be investigated.” He went on to say he didn’t want something that would “perceived” as a “partisan witch hunt,” but the signal was clear.

In 2009 Attorney General Eric Holder followed up by appointing John Durham as a special prosecutor to investigate CIA agents and contractors for their interrogations in the war on terror. Mr. Durham also looked into whether agents had illegally destroyed tapes of the interrogations. Mr. Durham never brought charges, but Mr. Obama’s call for a criminal probe was clearly aimed at indulging the left’s Bush hatred.