These are dangerous days for Stephen Bannon, President Trump’s brain. A new book about the White House chief strategist portrays the president as the empty vessel into which Mr. Bannon poured his ideology and agenda, propelling the two of them into the White House. The book, “Devil’s Bargain: Steve Bannon, Donald Trump, and the Storming of the Presidency,” by Joshua Green, a reporter who has known Mr. Bannon for years, is a best seller that gives Mr. Trump second billing. That’s made the empty vessel very angry.

Mr. Trump’s White House is drifting so dangerously that we find ourselves searching for ballast in unlikely places. There’s Jeff Sessions, who refused to resign his post as attorney general amid daily humiliations. The new chief of staff, John Kelly, got off to a good start by arranging the sacking of Anthony Scaramucci, the inept and mercifully short-lived communications director.

And then there’s — ready for this? — Mr. Bannon, the alt-right ideologue who’s emerged as one of the steadier hands on the ship. During the bile-filled, Breitbart-fueled campaign, Mr. Bannon encouraged Mr. Trump, who called him “my Steve,” to toss all convention overboard. Now, while Mr. Trump tweets and rages, and drifts aimlessly from one policy to the next, Mr. Bannon keeps a whiteboard in his office war room with a handwritten list of Mr. Trump’s campaign promises. Not many of these promises have been checked off. But what’s interesting is that Mr. Bannon is keeping such a list, and while it’s easy to disagree with a lot of the items on it, he at least seems to represent fealty to what Trump voters said they wanted.