As Senator Chris Murphy, Democrat of Connecticut, sat seething in his office last month watching President Trump blame diversity requirements at the Federal Aviation Administration for the deadly plane crash over the Potomac River, members of his staff warned him against publicly venting his rage.

The midair collision had happened less than 12 hours earlier, they reminded him; bodies were still in the water and families were still being notified about the deaths of loved ones. Perhaps it would be more befitting of a U.S. senator to be respectful of the tragedy and all of its unknowns, rather than seize the political moment and respond?

Mr. Murphy had no time for that.

“Everybody in this country should be outraged that Donald Trump is standing up on that podium and lying to you — deliberately lying to you,” he said in an impassioned video he recorded and posted within 30 minutes of Mr. Trump’s news conference. “Every single senator and member of Congress should call him out for how disgraceful it was.”

Many did, but none managed to do so quite as quickly or concisely as Mr. Murphy, 51, who has seemed to be everywhere, all at once, since Inauguration Day, staging a loud and constant resistance to Mr. Trump at a time when Democrats are struggling to figure out how to respond to him.

Mr. Murphy, a career politician who rose to national prominence as a gun safety advocate after the 2012 Sandy Hook elementary school shooting in Newtown, Conn., has emerged in the opening weeks of Mr. Trump’s second term as one of the most effective Democratic communicators pushing back against a president unbound.

In two-minute videos on social media, which he records from his office on Capitol Hill; an almost constant stream of posts on X; passionate floor speeches; and essays he writes on his Substack, Mr. Murphy is attempting to explain in digestible sound bites that what is happening in Washington is very simple: It’s a billionaire takeover of American democracy.