Justice David H. Souter, who died last week at age 85, made few public appearances after he retired from the Supreme Court in 2009. When he did, he stayed away from politics.

But a seemingly bland question from an audience member at a New Hampshire arts center in 2012 provoked an impassioned response from the justice, who was the opposite of excitable.

He said he was worried that public ignorance about how American government works would allow an authoritarian leader to emerge and claim total power. “That is the way democracy dies,” he said.

“An ignorant people can never remain a free people,” the justice said. “Democracy cannot survive too much ignorance.”

Not understanding how power is allocated among the three branches of government, he said, leaves a void that invites a strongman. After a crisis, he said, “one person will come forward and say, ‘Give me total power, and I will solve this problem.’”

That was four years before Donald J. Trump, as he accepted the Republican presidential nomination for the first time, said something strikingly similar: “Nobody knows the system better than me, which is why I alone can fix it.”