SAN FRANCISCO — Senator Bernie Sanders has long promised to run a different kind of campaign and be a different kind of president. On Saturday night, what was billed as his first “grass-roots fund-raiser” of the 2020 campaign was certainly different, as donor events go.

It was at a nightclub, for one thing. The D.J. was spinning a mix that included “Eye of the Tiger,” the underdog anthem from the “Rocky” movie series. Tickets were as cheap as $27, for another. There were food trucks out back (tacos and a Taiwanese-Korean blend).

The night had the vibe of a Sanders rally, only smaller, darker and more intimate, and with booze. Disco balls dangled from the ceiling. When the lights flashed briefly during his remarks, Mr. Sanders joked about the special effects.

“Very sophisticated,” he deadpanned. “Either that or someone was leaning on the light switch.”

People laughed. This was, after all, Mr. Sanders’s crowd to lose, and he did not lose them, despite some delays before taking the stage.