President Obama is said to be a reflective man, and often he is the one saying so, but you wouldn’t know it from his Thursday press conference with Canadian Prime Minister Justin Trudeau. Asked about political polarization and the Donald Trump phenomenon, Mr. Obama denied all responsibility. He doesn’t seem to appreciate the kind of country he will leave behind.

“What I’m not going to do is to validate some notion that the Republican crack-up that’s been taking place is a consequence of actions that I’ve taken,” Mr. Obama...