Progressive activists looking to push the California Democratic Party further to the left at the party’s annual convention this weekend may be making it harder to challenge Republicans in more conservative parts of the state, some party leaders worry.

A host of first-time convention delegates, many of them backers of Sens. Bernie Sanders of Vermont and Elizabeth Warren of Massachusetts, argue that the party needs to be more aggressive in pushing a progressive agenda that calls for single-payer health care, free college education and tougher environmental rules, including a ban on fracking.

Saturday’s convention vote for a new state party leader is “a fight for the heart and soul of the Democratic Party,” said one of the two main candidates, Kimberly Ellis of Richmond, a favorite of the Sanders wing whose campaign slogan calls for “redefining what it means to be a Democrat.”

But while the progressive platform draws enthusiastic applause from Democrats in the big cities and other deep-blue parts of coastal California, it’s far less popular in the Central Valley and other, more conservative, parts of the state, where the party is targeting Republican congressional incumbents in hopes of winning back the House next year.

“Bernie Sanders types wouldn’t do well in parts of the state,” said former San Francisco Mayor Willie Brown, who spent 15 years as the Democratic speaker of the Assembly and now writes a weekly column for The Chronicle. “You see candidates who win in San Francisco but wouldn’t have a chance in the (Central) Valley.”

The dispute comes at a time when the 3,000-plus delegates and guests to the Sacramento convention, which will run from Friday to Sunday, should be slapping high-fives at the growing strength of the Democratic Party in California.

Gov. Jerry Brown is a Democrat, as are all seven other statewide elected officials. The party holds a two-thirds majority in both the Assembly and Senate, making Republican members largely irrelevant.

In Congress, both senators are Democrats, and Democrats hold 38 of the state’s 53 House seats. Democrat Hillary Clinton received better than 61 percent of the vote in the November presidential election. Democratic registration is about 45 percent and growing slowly, compared with 26 percent and shrinking for Republicans.

Until a couple of months ago, Ellis led Emerge California, an organization dedicated to recruiting and preparing women to run for office. Now, she is running against party Vice Chair Eric Bauman to replace John Burton, retiring as party chair, and she says the Democrats have to make changes.

The party needs to become more progressive and less willing to be manipulated by the state’s corporate interests if it’s going to attract a new generation of voters, many of whom became politically active as part of Sanders’ unsuccessful run for president last year, Ellis said.

Burton, the 84-year-old former congressman and legislator who has run the state party for the past eight years, doesn’t see the problem.

“If you can find someone to the left of me, I’d like to meet him,” he said. “We’re probably the most liberal state party in the nation.”

It’s important to set the proper priorities, Burton added.

In years past, “the deal wasn’t about having a certain type of legislator, but having operational control” of Congress or the Legislature, he said. “A lot of times you can be holier than thou, but you don’t ask a guy to walk the plank” by forcing him to take a position that could hurt him in his district.

The election battle between Ellis and Bauman also reflects the lingering bad feelings between Sanders’ supporters and the Clinton backers who controlled the state party during the last election. One reason for the progressives’ anger, said former Sanders campaign surrogate Nina Turner, is that “nobody has fully addressed their concerns,” which include complaints that party organizations put their thumbs on the scale for Clinton.

Although Ellis backed Clinton last year, she has become a favorite of the Sanders faithful. In April, Our Revolution — the organization that grew out of Sanders’ presidential campaign — endorsed Ellis, joining National Nurses United, the Oakland-rooted union that was a key driver in the Vermont senator’s presidential run.

“She’s really going to open the party,” said Shannon Jackson, executive director of Our Revolution. “I don’t see her as an establishment type who is blind and deaf to the voice of the people.”

Bauman, 58, a Bronx-born nurse and political operative from Los Angeles, said Ellis and her backers are trying to fix something that’s not broken.

“We’re the only state party that knows how to do it right, that’s been able to win even in bad years like 2016,” he said. “It’s my experience and ability to know how to run a party that sets me apart.”

Both Bauman and Ellis say all the right things about party unity, calling for a big-tent approach that doesn’t require political ideology tests to determine who is and who isn’t a real Democrat.

Ellis, 43, says that because she and Bauman “agree on most of the issues,” the race is a contrast of tones and styles.

For her, that tone likely will be a sharper one, especially with party leaders who aren’t living up to the expectations she believes voters should have of Democratic politicians. She said she will be unafraid to challenge party leaders.

Since Democrats hold all statewide offices and have a supermajority control in the Legislature, Ellis wonders why they haven’t reformed Proposition 13, passed Medicare-for-all or banned fracking.

“We will call out those Democratic Party officials who have hedged on those issues, like the governor. He’s hedged on Medicare for all,” Ellis said. Under the Trump administration, she said, “we don’t have the luxury to playing it safe for what is right and fair and just.”

That’s a threat Democratic officeholders have to take seriously. Last year, for example, labor leaders and party activists challenged Democratic Rep. Ami Bera, whose suburban Sacramento district is a perennial GOP target.

Despite the district’s conservative tilt — and Bera’s tenuous hold on the seat — the Democratic left wing demanded the congressman take a more progressive stance on issues like trade, immigration and the environment. Some labor groups even endorsed Bera’s GOP opponent, Sacramento County Sheriff Scott Jones. In November, Bera won a 51-49 percent squeaker.

The prospect of having a new state party leadership that will give its imprimatur to similar left-leaning challenges to moderates is a daunting one for many Democrats. It’s a special concern when the national party is targeting seven of the state’s 14 Republican members of Congress, all of them in districts with conservative roots.

“One size doesn’t fit all,” said Nate Monroe, a political science professor at UC Merced. “You can’t say, ‘Here’s what every Democrat is saying and I got to say it, too.’”

Delegates will vote on the new party chair Saturday afternoon, with the results expected later that evening.

John Wildermuth and Joe Garofoli are San Francisco Chronicle staff writers. Email: jwildermuth@sfchronicle.com and jgarfoli@sfchronicle.com. Twitter: @jfwildermuth, @joegarofoli