At a rally in Erie, Pa., last weekend, Senator Bernie Sanders of Vermont laid out a bleak vision to voters.

“Working-class people are angry. And some of them are thinking about Trump,” he said, ticking off a litany of economic statistics. “The current system is broken!”

Mr. Sanders recounted tales of families living paycheck to paycheck, and described elites and the status quo as “disgraceful” and “disgusting.” He grew louder as he talked about the ravages of addictions to alcohol and opioids, reaching a shout as he got to what he called the worst addiction of all: “It’s greed!”

The crowd roared in agreement.

On the campaign trail on behalf of Vice President Kamala Harris, Mr. Sanders is embracing a dark tone in his outreach to frustrated working-class voters, giving voice to a grimly populist message that contrasts with her campaign’s upbeat optimism.

Mr. Sanders has stumped across swing states for months, and the 83-year-old independent said in an interview that with the campaign’s end in sight, he planned to hold rallies through Election Day. His fiery speeches aim to win over voters leaning toward former President Donald J. Trump by acknowledging working-class anger over the economy. Short of that, he hopes to motivate reliable Democratic voters to turn out.

“Workers’ rights are on the table,” Mr. Sanders said. “That’s something I can’t sit out, so I will do everything I can to see that Trump is defeated and Harris is elected.”