Peter Keisler, former head of the Justice Department's Civil Division and Acting Attorney General of the United States, writes in with the following important point about my piece yesterday on Trump's threat to jail Hillary Clinton:
I thought your piece on the Trump promise to appoint a special prosecutor was terrific. I had one thought that I wanted to share with you. In addition to the three important norms you explain such a statement violates, there’s a fourth: this wasn’t an action by a sitting President, but a campaign promise to take such action by a candidate. So on top of the fact that this would be a deeply improper action for any President to take, we also have the specter of someone running for office asking people to vote for him based on a promise to investigate and jail a particular person—having a national referendum over (in part) whether someone should be prosecuted. One could debate whether it’s better or worse to have prosecutorial decisions corrupted by the White House or by being subject to a popular vote, but worst of all is to have both.
Keisler is absolutely right and I'm a little embarrassed that I didn't think of this fourth point myself. I am amending the original post to include it.