When Gavin Newsom stepped in front of a camera Tuesday evening, he was a California governor addressing his constituents as a crisis of civil disorder gripped pockets of Los Angeles.

But by the time Mr. Newsom was finished, it was clear the governor was speaking not only to his state but to his country, and his beleaguered party.

To Democrats looking for direction and leadership, Mr. Newsom used one of the highest profile moments of his political career to lay out the threat he argued President Trump posed to the nation, and how Americans should resist it. And he suggested he was the man to lead that fight.

“Democracy is under assault right before our eyes,” Mr. Newsom said. “The moment we’ve feared has arrived.”

The next presidential election is more than three years away. But it was hard to watch the speech, delivered as anti-Trump protests spread from Los Angeles to other cities, and not wonder if the 2028 campaign had already begun. This may well prove to be a turning point both for Mr. Newsom and for the Democratic Party.