WASHINGTON — President Trump looked out on a chamber divided, turning often to his left, where the cheers sustained him.

Already, the room had heard his ubiquitous pledge to restore bygone American greatness, completing its unlikely journey from trucker hat slogan to presidential tagline before a joint session of Congress.

That much, most Democrats could abide, silently and seated. The president’s follow-up would not fly.

“We have begun to drain the swamp of government corruption,” Mr. Trump said, gazing sternly ahead.

Some Republicans shot to their feet. Democrats began to laugh, loudly enough for a nation to hear.