Randy Villegas’s campaign for Congress in California would feel right at home in a liberal district. A self-described populist who supports Medicare for all, he proudly promotes his endorsements from progressives including Senator Bernie Sanders and Representative Alexandria Ocasio-Cortez.

But Mr. Villegas is not running in a Democratic bastion like San Francisco or Los Angeles. Instead, he’s mounting an ambitious campaign in California’s conservative Central Valley against Representative David Valadao, a battle-tested Republican.

Republicans have eagerly seized on Mr. Villegas’s candidacy, deriding him as a socialist who is out of step with the district while secretively spending money to boost his primary campaign over a more moderate Democrat to ensure that Mr. Valadao winds up with what they appear to consider an easier opponent.

Mr. Villegas is unfazed.

“What we have right now is a populist message that is resonating across the board,” he said in an interview, insisting that his message of refusing corporate dollars and fighting for universal health care transcended party lines and was hitting home in his battleground district. After all, he said, “who you voted for in the 2024 election” was irrelevant to health challenges such as valley fever, diabetes or cancer.

As Democrats fight for control of Congress, prominent left-wing politicians including Mr. Sanders of Vermont and Ms. Ocasio-Cortez of New York are increasingly inserting themselves into primary races to elevate progressives in competitive battleground districts.

That’s a departure from the progressive playbook of years past, which generally focused on backing candidates in deep-blue turf where campaigns tended to focus more on liberal ideology and less on electability in general-election contests.

The effort aims to rebut the conventional wisdom that running moderate Democrats who appeal to centrists and Republicans with middle-of-the-road policies is the best strategy in competitive races. Now, progressives are saying that candidates with policies aimed at helping working families and critiquing the wealthy can win anywhere.

Running candidates who “have the guts to stand up for the working class,” Mr. Sanders said in an interview, should be a “winning formula in almost every part of the country.”

This year, Mr. Sanders has backed a slate of candidates in competitive House and Senate races who promote working-class bona fides and espouse populist policies, even if not all describe themselves as progressive.

The list includes Sam Forstag, a smoke jumper in Montana; Bob Brooks, a retired firefighter in Pennsylvania; Brian Poindexter, an ironworker in Ohio; Rebecca Cooke, a small-business owner in Wisconsin; Abdul El-Sayed, a former county health director in Michigan; and Graham Platner, an oysterman in Maine.

In the competitive districts that also featured fierce Democratic primaries, some Sanders-backed candidates have already prevailed over moderate opponents this spring, including Mr. Poindexter, Mr. Platner and Mr. Brooks.

Of Mr. Sanders’s 16 congressional endorsements so far this election cycle, seven are in races considered at least somewhat competitive — a departure from his four endorsements in competitive congressional races out of 35 total in the 2024 and 2022 election cycles.

Jeremy Slevin, a senior adviser to Mr. Sanders, acknowledged that the senator’s strategy had shifted this year. It was influenced, Mr. Slevin said, by the success of Mr. Sanders’s “Fighting Oligarchy” rallies last year in conservative regions, which emboldened his view that his ideology resonated widely.

Mr. Sanders will rally for Mr. Platner in Maine this weekend. Ms. Ocasio-Cortez, who declined to comment, will appear with Mr. Forstag next week at a rally in Missoula, Mont.

The strategy is not without its detractors.

Matt Bennett, the executive vice president of Third Way, a centrist Democratic think tank, argued that the track record of successful Democratic campaigns showed that moderate politics were the correct formula in competitive districts.

“This notion that tacking sharply to the left is going to bring out this mythical band of voters who are just waiting to be mobilized by fairly radical ideas is a complete fantasy,” he said.

Mr. Bennett said he feared a scenario where a progressive in a competitive general election race became an easy, “unelectable” target for the right — pointing to 2018 losses by Kara Eastman in Nebraska and Dana Balter in New York, both progressives who fell short against Republicans in crucial swing districts.

Progressives, for their part, still harbor grudges over past instances when Democrats eschewed candidates from the left who might have won with more party support.

Bill Hyers, a progressive Democratic strategist, remembered a battleground 2022 Oregon contest in which establishment PACs responsible for electing Democrats to the House chose not to spend heavily to help Jamie McLeod-Skinner, a progressive who had upset an establishment Democrat in a primary. Ms. McLeod-Skinner ended up losing to her Republican opponent by just two percentage points.

The left flank of the party, Mr. Hyers said, has grown tired of seeing the traditional “generic white guy” candidate fail to excite voters and lose elections.

“It’s not working anymore, so why would we sit aside while you continually screw something up?” he said. “How about we have an agenda, say what we’re going to do, and have people who are real people?”

Mr. Forstag acknowledged that many of the voters he’s courting in western Montana — where Representative Ryan Zinke, a Republican, is retiring — are not supporters of Ms. Ocasio-Cortez. But he said her core beliefs on affordable housing, health care and child care would resonate across party lines.

“We do not have to agree on every single issue,” he said in an interview.

Mr. Forstag, a union leader, spent four years as a smoke jumper, a specialized type of firefighter for the U.S. Forest Service who parachutes in to fight wildfires. He said his inspiration for running came last year when thousands of Forest Service employees were fired as part of the Trump administration’s push for government efficiency. He spoke at the “Fighting Oligarchy” rally last year in Missoula, where Mr. Sanders and Ms. Ocasio-Cortez drew nearly 7,500 people.

Mr. Forstag dismissed the idea that moderate candidates were Democrats’ best options in red or purple districts.

“A lot of the people that Democrats have run across the country have been losing,” he said. “So we need to change something.”

He faces several Democratic primary opponents, including Ryan Busse, a former nominee for governor, and Matt Rains, a rancher and army veteran who is running as a moderate.

Mr. Rains argued that moderate Democrats had the better track record of courting swing voters in Montana, where, this year, the eventual Democratic nominee is likely to face Aaron Flint, a radio host who is leading the Republican primary.

Democrats aligning with progressives such as Ms. Ocasio-Cortez, Mr. Rains said, offered Republicans fodder to paint Democrats as far-left radicals.

“We don’t need to give them more ammunition to do that,” he said. “It feels like we’re going to shoot ourselves in the foot by leaning way too far to the left and not being able to identify with the average Montanan.”

In California, Mr. Valadao’s Central Valley district includes Bakersfield and some liberal areas that Democrats added during redistricting to make it more competitive. But it is still a contest with a razor-thin margin, and Dr. Jasmeet Bains, a state assemblywoman running against Mr. Villegas in the Democratic primary, argues that she is the better fit for the region.

Ms. Bains is known as a Valleycrat — a term for a more moderate Democrat in the Central Valley — and has slammed Mr. Villegas as “Radical Randy,” saying he has “endorsed socialist-run health care.”

Ms. Bains declined an interview request. Her campaign provided a statement highlighting her local roots and declaring that “the Valley doesn’t care about party labels, and they deserve better than politicians who only offer empty promises.”

Mr. Villegas called Ms. Bains “Republican-lite,” noting that both she and Mr. Valadao had accepted money from corporate donors. “It’s not even this fight about left versus right — it’s bottom versus top,” he said.

The Democratic establishment put its thumb on the scale in some of these races earlier this month, when the Democratic Congressional Campaign Committee endorsed candidates in contested primaries. It aligned with Mr. Sanders in some places, endorsing Mr. Brooks in Pennsylvania. But it also backed Ms. Bains.

Mr. Sanders dismissed the group’s involvement.

“The establishment Democrats live in their world — they collect a lot of money from wealthy people,” he said. “We live in a different world.”

Taylor Robinson and Leo Dominguez contributed reporting.