Here’s one sign Sen. Kamala Harris’ presidential rivals don’t think she has the California primary locked up: Thirteen of them will be attending the state Democratic Party convention starting Friday in San Francisco, trying to claim a stake in her home turf — the party’s largest electoral prize.

They haven’t been scared off by Harris winning statewide elections three times, locking up endorsements from California’s top elected officials, racking up $13.2 million in campaign contributions, spending more on digital ads than nearly every other Democrat, and starring in one of the season’s most impressive spectacles — her January rally in Oakland before an estimated 20,000 people.

Instead, they’re looking at polling that showed her running third in California among Democratic voters with 17%, behind former Vice President Joe Biden (26%) and independent Vermont Sen. Bernie Sanders (18%). Nationally, after a burst of attention following the Oakland rally, she has settled in at around 8%.

“I don’t know why she’s not caught fire. But she hasn’t,” said Tim Malloy, assistant director of the Quinnipiac University Poll. “I think everybody is sampling and taking a look at everybody. But for now, she’s a regional candidate. A California candidate.”

White House hopefuls know they can win delegates in California’s March 3 primary even if Harris does well, because this isn’t a winner-take-all state. More than half of California’s 495 voting delegates are allotted by congressional district, so candidates can pick off a few delegates if they win at least 15% of the vote in a district or statewide.

But the biggest reason so many Democrats will be in San Francisco next weekend is the lesson learned from recent presidential campaign history: Anything could happen.

“Clearly, nobody is scared of doing anything in this race — that’s why we have 2,000 candidates,” said Melissa Michelson, a professor of political science at Menlo College.

“It’s more of a reflection on the lessons of 2016: Nobody knows what will happen, especially in a big field,” Michelson said. “Four years ago, we were talking about Jeb Bush and Marco Rubio.”

Joining Harris at the party’s convention will be more than half the announced Democratic field: Sens. Cory Booker of New Jersey, Kirsten Gillibrand of New York, Amy Klobuchar of Minnesota, Elizabeth Warren of Massachusetts and Sanders; Pete Buttigieg, mayor of South Bend, Ind.; Reps. Eric Swalwell of Dublin and Tulsi Gabbard of Hawaii; former Reps. Beto O’Rourke of Texas and John Delaney of Maryland; former Colorado Gov. John Hickenlooper and current Washington Gov. Jay Inslee; and former Housing and Urban Development Secretary Julián Castro.

One big name not expected to be here is Biden. But that’s more of a front-runner’s decision not to get lost in the scrum.

Many of the candidates who will be here are hovering in the low single digits in the polls. This is their chance to remind the national media that they exist, tap into the country’s nation’s largest trove of Democratic donors, and rally volunteers to do the yeoman’s organizing work before the primary.

“We’re campaigning in every state,” said Mike Hopkins, a spokesman for Delaney, one of the 1 percenters in the polls. “Sen. Harris is certainly one of the senators from this state, but she’s got to earn their vote like everyone else. I think all Democrats should remember the lessons we learned in 2016 about expectations.”

Lisa Tucker, chief strategist for Swalwell, said she doesn’t think it’s a reflection on Harris that so many rivals will be here.

“It’s California. It’s the largest state. It’s delegate-rich,” Tucker said. “With so many candidates, we all have plans on how we can win something. Nobody is ceding any state to anyone.”

Many of the candidates plan to pop into town to speak to the full convention, address party caucuses that focus on labor, environmental and other issues, perhaps attend a fundraiser and then head off to other early primary states.

At least seven — Booker, Castro, Gillibrand, Harris, O’Rourke, Sanders and Warren — plan to appear at a MoveOn forum Saturday in San Francisco. Warren plans to hold a town hall meeting at 7:30 p.m. Friday at Laney College in Oakland, and Sanders has scheduled a rally for 5 p.m. Saturday at Arena Green East in San Jose.

For Sanders, an independent who is unlikely to have much help from the Democratic Party, such rallies are designed to help recruit the volunteer army he’ll need to compete in the primary. Sanders has spent four years building a formidable grassroots force in California, where he won 45% of the vote and 206 delegates in the 2016 primary.

Complete convention coverage At least 14 Democratic presidential candidates are coming to the Bay Area for the state party’s San Francisco convention Friday through Sunday. Check SFChronicle.com for complete coverage of the candidates and “It’s All Political” podcasts, and go to http://bit.ly/2wjAlc9 to receive special editions of the Political Punch newsletter.

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Since the launch of his 2020 run, 384,000 Californians have taken some kind of action for Sanders, his campaign says — donating money, volunteering or hosting or attending an event.

“We’re continuing to build a volunteer force unlike any other candidate,” said campaign manager Faiz Shakir. “You have to to do that in order to win. California is too big. You could spend a billion dollars on TV and it wouldn’t be enough. ”

Nevertheless, Shakir said, “it will be an uphill fight against establishment candidates like Biden and Harris.”

Harris will bring many home-field advantages to the convention and the primary. On Friday, she will attend a fundraiser in San Francisco hosted by billionaires Ann and Gordon Getty and attended by Gov. Gavin Newsom, who has endorsed her.

It’s a sign of Harris’ familiarity with the state’s deepest pockets. Of the $13.2 million she raised through March, $4.3 million came from California, according to federal campaign reports.

That was before Biden got into the race. The No. 2 Democratic fundraiser in California was Booker, at $1.2 million, followed by Sanders with $785,000. President Trump raised $1.4 million.

Unlike most of the other candidates, Harris won’t have to spend her precious few minutes in her convention speech introducing herself to party insiders. They’ve known her since she first ran for San Francisco district attorney in 2003 and backed her through her statewide wins for attorney general and the Senate.

Instead, she will focus on the economic plans she has proposed: a $6,000 tax break for low- and middle-income earners; a system to fine companies that pay women less than men for comparable work; and a $13,500 annual pay hike for teachers. Harris will point out how these proposals would put money in people’s pockets now — unlike other candidates’ soak-the-rich proposals that could take years to help.

Harris won’t take anything for granted, spokesman Ian Sams said.

“Her native status will be helpful,” Sams said, “but we are under no illusions that anything is locked up.”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli