WASHINGTON — One recent afternoon, Senator Bernie Sanders of Vermont gave another of the populist speeches that have drawn the largest crowds of the 2016 campaign to his rallies around the country and have made him the unexpected rival to Hillary Rodham Clinton for the Democratic presidential nomination.

The role of “super PACs” is “corrupt and amounts to legalized bribery,” he bellowed. Waving his arms, he quoted Abraham Lincoln and shared his own “vision for the future of this country.”

On the campaign trail, the speech would have elicited wild enthusiasm from his liberal supporters. But this was the Senate, which was virtually empty except for Senator Dianne Feinstein, Democrat of California, who was busy editing her own speech, and Senator Mike Lee, Republican of Utah, who was texting.

“You come here, it’s like, ‘O.K., not much response,’ ” Mr. Sanders said with some resignation in his Senate office earlier this month..