BETHLEHEM, PA—On Saturday afternoon, the Stabler Arena at Lehigh University in Bethlehem, Pennsylvania, had the energy of a Taylor Swift concert. There were high fives, tears, and the sound of 5,000 attendees rising from their chairs for a glimpse of their rock star. Boomers leaned over the metal barricades, hands outstretched. A brunette in a “Protect Trans Kids” shirt sent a sharp whistle soaring through the arena.

All this fanfare was for an 83-year-old democratic socialist from Vermont: Senator Bernie Sanders. On the 19th stop along his Fighting Oligarchy tour, a series of rallies raging against “oligarchs and corporate interests,” the senator, now a seasoned performer, shuffled onto the stage just as the beat of John Lennon's “Power to the People” dropped.

“Thank you, Lehigh Valley,” he boomed in his signature Brooklyn accent. “This is not only a great turnout, but you are loud.”

A pause, and then a grin: “I think Trump is hearing you in Mar-a-Lago, and he wants to turn down the volume—but we ain’t gonna let him.”

The president is why Sanders was devoting his Saturday to speak in this postindustrial hub that’s trying to reinvent itself.

With the Democratic Party in crisis after its comprehensive defeat last November, Sanders is crisscrossing the country to pitch a new vision—one that he hopes can compete with Trump’s brand of populism.