TEMPE, Ariz.—When President Trump and billionaire ally Elon Musk began to drastically downsize the federal government , Sen. Bernie Sanders called his top adviser, Faiz Shakir , and suggested they hit the road. “I have a feeling that people are pretty angry out there,” he said.

They initially reserved a space at their first stop of Omaha, Neb., that could hold 800 people, but had to move to a bigger venue so 3,400 could attend. As the tour headed farther west this past week, the political independent who caucuses with Democrats said he was shocked by crowds that exceeded even his 2020 presidential campaign rallies. About 15,000 people showed up to see him in Tempe on Thursday night, filling the arena and an overflow room, with thousands more waiting outside.

U.S. Representative Alexandria Ocasio-Cortez (D-NY) speaks during a “Fighting Oligarchy” rally with U.S. Senator Bernie Sanders (I-VT) in Denver, Colorado, U.S. March 21, 2025. REUTERS/Kevin Mohatt

“That is insane, I’m not running for anything,” said the Vermont senator and two-time presidential candidate in an interview. “People are outraged and they’re frightened and they want to fight back. And this is one form of beginning the struggle to fight back.”

As Democrats tussle with each other on Capitol Hill and struggle to find a strategy to combat Trump’s fast-moving agenda, Sanders is stepping into the void and pitching his prescription for how they can start winning again. The Democratic caucus might look dead on its feet in Washington, but in Arizona voters are fired up, at least for a night. And colleagues are starting to borrow from his playbook.

With the words “FIGHT OLIGARCHY” projected all over the hockey arena, Sanders and his progressive protégé Rep. Alexandria Ocasio-Cortez (D., N.Y.) railed Thursday against Trump and Musk. It was a vintage Sanders performance, complete with his surveying the crowd for examples of people living paycheck to paycheck and calling for free college and universal healthcare. But Sanders was angrier and had a longer list of what he deemed as threats—as well as a ripe new target in Musk.

Addressing Trump, Sanders said: “I’m not going to allow you and your friend Mr. Musk and the other billionaires to wreak havoc on the working families of this country.”

Sanders, 83, has gone after billionaires for decades, and now one of the richest men in the world is leading Republican efforts to cut the federal workforce. The blowback has sparked new enthusiasm for the self-described democratic socialist who pushed for a vast expansion in the U.S. safety net. The broader question facing the left headed into the next election is whether Sanders’s brand of economic populism is a path forward for drifting Democrats—or an energetic but politically dicey sideshow.

U.S. Senator Bernie Sanders (I-VT) speaks as U.S. Representative Alexandria Ocasio-Cortez (D-NY) reacts during a “Fighting Oligarchy” rally in Denver, Colorado, U.S. March 21, 2025. REUTERS/Kevin Mohatt

In the interview, Sanders said the Democratic Party has lost its way. It is strong on women’s rights, civil rights and gay rights, Sanders said. “But in terms of fighting for the working class of this country, Democrats have been virtually nonexistent,” he added. “They haven’t been there.”

The key to understanding the political moment is to look at it through a class-based lens, not an ideological one, said Shakir, who managed Sanders’s 2020 presidential campaign. Rather than left versus right, the correct framework “is to think of the top versus the bottom, particularly the very top,” he said.

Polls show Democrats’ popularity at a new low. With little power to block Republicans’ agenda and beset by internal disagreement, the party is looking toward the 2026 midterms as a chance to get back on track.

The “Fighting Oligarchy” tour comes on the heels of a painful period for Democrats as they wrestled with whether they would be willing to use a government shutdown as leverage against the Republicans. The infighting culminated in the decision by Senate Minority Leader Chuck Schumer (D., N.Y.) and several other Democrats to vote with Republicans to avert a shutdown, a move that infuriated progressive voters. Some suggested that Ocasio-Cortez should challenge him in 2028 for his Senate seat.

There are signs that Trump’s cuts are hitting home. An NBC poll earlier this month found that 27% of registered voters said they or someone they know has been hurt by the Musk-led Department of Government Efficiency.

The people who show up for Sanders and Ocasio-Cortez are a reminder “there are still others out there who say, ‘This isn’t right, there’s something that has to be done,’” said Rhys Kapchan, 22, a student from Gilbert, Ariz., who attended the Tempe rally.

In her own remarks, Ocasio-Cortez said Republicans are stealing Americans’ healthcare, Social Security and veterans benefits to pay for tax cuts for billionaires and “bailouts for their crypto friends.” She said it was easy to feel despair and to give up but Americans shouldn’t. One attendee shouted: “We won’t do that.” Another yelled: “We’re here.”

A recent open-ended CNN poll asked respondents who best represented Democrats’ core values. Sanders was third at 8%, with former Vice President Kamala Harris second at 9% and Ocasio-Cortez first at 10%.

Republicans say Sanders and Ocasio-Cortez are liabilities to the Democratic Party.

“The Democrats’ new leaders, Bernie Sanders and AOC, are staging the latest political theater, determined to distract from the Democrat Party’s failed record and crumbling brand,” said Emily Tuttle, spokeswoman for House Republicans’ campaign arm. Americans “won’t be fooled by this transparent attempt to push the far-left, radical agenda,” Tuttle said.

Still, other congressional Democrats are echoing Sanders’s message. In town halls and events across the country this past week, centrists and progressives alike focused on how the firings of federal workers, combined with the Trump budget agenda that is winding its way through Congress, could make ordinary Americans’ lives worse by curtailing government services—from Medicaid and Social Security to the Department of Veterans Affairs and the Education Department. Democrats tell voters that GOP spending cuts are paying for tax relief for wealthy people like Musk.

“All of this stuff that you’re hearing about every single day is so they can give a big giant tax cut to people who don’t need a tax cut,” said Sen. Mark Kelly of Arizona, a centrist Democrat, at a town hall in Scottsdale. “We could raise the taxes of billionaires and they will still be billionaires, and that’s what we should be doing,” he said at the event, which was focused on potential Medicaid cuts.

“People want authentic, unfiltered, unscripted conviction,” said Ben Wikler, who is chair of the Wisconsin Democratic Party and ran in a dark-horse campaign to lead the Democratic National Committee.

Sanders has always been able to draw large crowds of voters unhappy with the political status quo, but his electoral success has been limited. His own presidential bids were squashed by the party’s establishment wing and concerns about the price tags of some of his proposals. During Harris’s 2024 bid for president, Republicans sought to remind voters that she had previously embraced Sanders’s Medicare for All policy before distancing herself from it.

Now Sanders’s message, which focuses on economic issues rather than hot-button social concerns such as transgender rights, is back at the forefront of Democratic politics. A rally Friday in Denver drew 34,000, his campaign said, the biggest Sanders event ever. His rallies show the “widespread and growing grassroots opposition to Donald Trump’s Golden Age for the oligarchs, and that’s a very good thing,” said Schumer in a statement.

“I’m noticing,” a smiling Sanders said about other Democrats who adopt his populist message. “My colleagues are not dumb people. They are, by and large, very, very smart. And you’ve got to be deaf, dumb and blind not to analyze the solution,” he said.

Write to Eliza Collins at eliza.collins@wsj.com , Lindsay Wise at lindsay.wise@wsj.com and Xavier Martinez at xavier.martinez@wsj.com