California is the largest prize in the calculations of any Democratic presidential candidate, and Bernie Sanders has been working the state for months, worrying his rivals.

Mr. Sanders has been organizing intensively among Latinos and young voters, producing campaign materials in seven languages, going, as one aide said, "where most candidates don't go." Mike Bloomberg has tried to counter Mr. Sanders with saturation advertising, including buying time at television stations in Arizona, Nevada, and Oregon that also reach California. Pete Buttigieg held three public events in the past week to capitalize on his early state momentum. Joe Biden and Elizabeth Warren remain competitive.

The attention reflects a growing concern among Mr. Sanders' rivals that if he performs well enough in the state, with its 415 delegates at stake on Super Tuesday, March 3, that he could build a delegate lead that is difficult to catch.

"California's one of those unique places because these presidential elections don't play out here very often," said Ace Smith, one of the state's best known political strategists. "There's just a real thirst."

Competing in the state isn't simple; it is home to some of the nation's most expensive media markets, there are roughly 20 million voters, and delegates are awarded both on the statewide level and in each of the 53 congressional districts. A candidate must hit 15% statewide to get a share of 144 delegates. Another 271 awarded by reaching 15% in a congressional district, with heavily Democratic districts offering more delegates.

Mr. Sanders' campaign has long counted California as important, deploying more than 80 staff here last year and sending Mr. Sanders regularly. He'll hold two rallies Friday in heavily Latino areas, on top of an event earlier this week in the San Francisco Bay Area, a Democratic stronghold rich with delegates.

He's running television ads in every market. Campaign staffers were out just days after ballots dropped on February 3rd, knocking on doors offering to collect them, a legal practice in California, and his events have booths set up to collect them. He is trying to show that organizing can be more potent that TV ads.

Mr. Smith, who ran Hillary Clinton's 2016 operation in the state, said the key question for Mr. Sanders is how high his support can go. If it hits close to 40%, it will be harder for multiple other contenders to win delegates, allowing him to run up the score.

Recent polls show Mr. Sanders in front of other top candidates in the state, with Mr. Biden, Mr. Bloomberg, Mr. Buttigieg, and Ms. Warren all hovering near the threshold for receiving delegates.

Rafael Návar, Mr. Sanders' state director, said the campaign believes he will win delegates in every congressional district.

"We've prioritized where most presidential campaigns don't go," he said.

Mr. Bloomberg, meanwhile, is blanketing every single television market in the state with ads, in addition to those out-of-state markets that reach California viewers. He is also buying advertisements in weekly newspapers in rural areas, hoping to hit voters who may not be getting much communication from Democrats.

He last campaigned in the state on Feb. 3, the day of the Iowa caucuses, traveling from the state capital of Sacramento to Fresno, in the state's farm belt, and ending the day outside Los Angeles. Dan Kanninen, the campaign's states director, said Mr. Bloomberg is similarly trying to win delegates in every district.

"It's premature to put a number on what we hope to get," Mr. Kanninen said, but he warned Mr. Sanders could run away with delegates if non-Sanders voters don't consolidate behind a single alternative. "Voting for a candidate who's going to get 5% or 6% does have the danger of creating the scenario around that threshold that could get Bernie a lead that's almost insurmountable."

Mr. Bloomberg's advertising is buoyed by roughly 300 staff members on the ground, by far the most of any campaign, led by strategists with deep California experience. The campaign will have held 1,000 organizing events in the state by March 3, spokesman Mike Buckley said, including niche get togethers like "Surfers for Mike" and "Scientists for Mike."

California is also home to some of the country's biggest Democratic donors. Mr. Bloomberg isn't taking any campaign contributions, but he's set up "leadership committees" of would-be donors who hold events akin to fundraisers where people can learn from Bloomberg allies about his campaign plans and policies.

Mr. Buttigieg in particular has done well with the California donor class; he's regularly held fundraisers in Hollywood and has raised nearly $10 million from California donors, more than from any other state. His challenge is to translate that support to votes.

He visited Sacramento and the farming city of Turlock last week and spent Thursday at a televised town hall in the Los Angeles media market. For candidates that can't afford to blanket the airwaves with ads, earning free media through campaign events is critical, said Mr. Smith, the California strategist.

Mr. Buttigieg's campaign is holding volunteer organizing events in at least 47 congressional districts this weekend, spokesman Ben Halle said. He declined to say which six districts haven't yet been organized.

Mr. Buttigieg's campaign has sent out a memo warning of a Super Tuesday scenario where Mr. Sanders dominates. And he, like Mr. Bloomberg, is arguing he's the single best candidate to go head-to-head with Mr. Sanders. Both have urged the other to drop out.

Mr. Biden, meanwhile, has only held public events twice in the state since November and has no television advertising, though he has a digital buy. He's more urgently focused on reviving his struggling campaign in Nevada and South Carolina, which vote next.

Ms. Warren similarly has spent no time in the state this year, though her campaign is hosting multiple events targeting Latino voters this week and has more than four dozen staff members. A spokesman declined to say if she plans to run TV ads. Amy Klobuchar has virtually no campaign infrastructure in the state, and her campaign just announced a seven state Super Tuesday ad buy that does not include California. Tom Steyer, the race's other billionaire and a California resident, is also up on the airwaves.

So far, just 8% of Democratic mail voters have returned ballots, according to tracking by Political Data Inc. Just a fraction of the state's 5 million independent voters have requested the ability to vote in the Democratic primary, prompting the Sanders campaign to schedule a Friday press conference to highlight and explain the process.

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Roughly 5% of Latino voters who vote by mail have returned ballots, and less than 5% of those ages 18 to 34 have, according to Political Data Inc.'s tracking. Both are key demographics for Mr. Sanders campaign.

This story was reported by The Associated Press.