Remember, back in the day, when newly elected presidents enjoyed a honeymoon? Donald Trump isn’t getting one. His adversaries are in combat mode, as if the campaign never ended. Demonstrations are planned. Money is pouring into the coffers of liberal groups. Activists are calling for an ongoing “resistance” movement. Mr. Trump is getting a taste of his own medicine because he, too, is behaving as if the campaign never ended. For weeks, he has continued to lob provocative tweets about his critics and the media, derided people who supported Democratic nominee Hillary Clinton, reveled in his election victory and took a victory...