President Trump plunged into a new year of work and partisan warfare Wednesday after 12 lonely shutdown days in the White House, pressuring Democrats for a deal on border security and reasserting his superiority on everything from the military to lawmakers who cross him.

During a more than 90-minute public Cabinet meeting, Mr. Trump delivered at times a stream-of-consciousness monologue on the state of world affairs as if he’d been bottled up without a live audience during the holiday-season isolation. He predicted the partial government shutdown could last “a long time” if Democrats who are taking control of the House on Thursday don’t cooperate with him.

“We have a long way to go and we want to solve the problems, not just sit back and enjoy the presidency,” Mr. Trump said.

He conducted the entire Cabinet meeting seated in front of a movie-like poster of himself in a “Game of Thrones” genre on the mahogany table. It proclaimed “Sanctions are Coming.”