Rzewski pieces like “Stop the War!” and “No More War” make no bones about their composer’s politics. But they’re not pamphlets in sound. They have wit, sensitivity. One of the dozens of what he calls “miles” that make up Mr. Rzewski’s sprawling solo-piano series “The Road,” “Stop the War!” bristles with crushing chords, and its performer even shouts out the title at one point. But the music keeps receding, as if stunned by the violence and rage it feels compelled to depict.

The son of pharmacists, Mr. Rzewski was born in Westfield, Mass., and educated at Harvard and Princeton, centers of mid-20th-century composition. He moved to Europe, became an important new-music interpreter, and in 1966 helped found the ensemble Musica Elettronica Viva, which used widely available electronic instruments for exercises in collaborative improvisation.

He now speaks a bit ruefully of those heady days. “Free improvisation was going to change the world,” he said of his generation’s 1960s dreams. “It was going to create an entirely new language, so that people could come together from different parts of the planet and instantly communicate.”

He paused. “Well, of course, we were wrong.”

But he hasn’t given up the fight. “This whole Black Lives Matter movement is very important,” he said. (Of course, ever the good socialist, he drew a distinction: “It’s not a party, it’s a movement.”)

His influence can be felt in pieces by much younger composers, like “sweet light crude,” a brooding mock love song to fossil fuels by David T. Little, who has also performed Mr. Rzewski’s music. When Andrew Norman, in his “Split,” used a piano concerto to reflect on power, on how a group (the orchestra) can exert control over an individual (the soloist), the effect was Rzewskian, even if the sound world was not. Ted Hearne’s WikiLeaks oratorio “The Source,” a driving and simmering reflection on the Iraq war that broods without settling for easy answers, feels as if it were in Mr. Rzewski’s lineage.

Mr. Rzewski practices the progressive ideas he preaches, making his scores available online and encouraging, rather than blocking, the dissemination of his recordings on YouTube. He remains, he says, a revolutionary optimist. Asked if it’s possible actually to affect politics through music, Mr. Rzewski answered, “Probably not,” before adding, with a wry smile: “But you have to write as if you could. You can’t be sure. You might.”