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RICHMOND — Bernie Sanders may be America’s most famous political independent, but as he rallied his Bay Area supporters on Monday, he sent his California fans who’ve also spurned political parties an urgent message: register Democratic.

“Please change your registration to Democrat,” the Vermont senator told more than 10,000 people who packed into a cavernous former Ford assembly plant on the edge of the San Francisco Bay. “Please do everything you can to get a Democratic ballot so that we can win here in California.”

The message reflects a potential pitfall for Sanders in California’s Super Tuesday presidential primary, despite his strong lead in the state’s polls.

More than 15 million of California’s roughly 20 million registered voters received their ballots in the mail earlier this month. But voters registered without party preference had to specifically request a Democratic presidential ballot to receive one — and only about 9 percent of them did, according to Political Data Inc., a prominent nonpartisan political firm in the state.

That means that millions of independent voters have received a ballot without any presidential candidates on it. While they can still request a Democratic presidential ballot from their county registrar or ask for a Democratic ballot if they vote in person, it’s a task many might forget to do before election day on March 3.

No other Democratic candidate will be more affected than Sanders, who does especially well among independents. And independent Latino and young voters — two groups that are key to Sanders’ base in California — are even less likely to have requested a Democratic ballot, with less than 5 percent who vote by mail doing so.

Those numbers are “a real concern” for Sanders’ campaign, said Jane Sanders, the senator’s wife and closest adviser, in an interview with the Bay Area News Group on Monday.

“We’ve done as much as we can in terms of communicating to people that we’d like them to register as Democrats” or request a Democratic ballot, she said.

Volunteers were ready to collect ballots from Sanders supporters to turn them in at his Richmond rally Monday, and the campaign has also established a hotline for supporters to call if they have any issues voting.

The Sanders fans who packed into the Craneway Pavilion — where Ford cars were built in the 1930s before the factory switched to churning out tanks during the second World War — said they were eager to cast their votes for the senator.

Richard Valencia, a 28-year-old film production employee from Pleasant Hill, brought his mom, Romana, an immigrant from the Philippines, to her first ever political rally.

“He feels like the first political candidate to ever speak for the people and not for himself,” Valencia said of Sanders. “For all the years of his career, he’s been giving the same message — and now it’s his prime time.”

In a 30-minute speech that elicited cheers from his audience after almost every line, Sanders argued for progressive priorities like Medicare-for-all, criminal justice reform, immigrant rights and the Green New Deal.

“Donald Trump, he thinks that climate change is a hoax,” Sanders said. “I think Donald Trump is a hoax. Under our administration, we’re going to listen to climate scientists, and not right-wing extremists.”

Sanders also went after former New York City Mayor Mike Bloomberg, who has spent more than $400 million of his own money to blanket the airwaves in California and other Super Tuesday states with ads promoting his candidacy.

Bloomberg has a right to run for president but “he does not have a right to buy the presidency,” Sanders declared. He blasted Bloomberg’s past support of the New York Police Department’s stop-and-frisk policy and his past opposition to raising the minimum wage — both positions that Bloomberg has changed — and said that “the American people are tired of billionaires buying elections.”

After years as a political underdog, Sanders and his supporters seemed energized by his status as a front-runner for the Democratic nomination following his win in the New Hampshire primary and close second place finish in the Iowa caucuses. He said the party’s establishment was “trembling” and “crying on television” about his success.

“The Democratic establishment is getting nervous,” Sanders said to huge cheers. “I hear the establishment saying, ‘oh, Bernie can’t win the election.’ Take a look at this crowd today!”

The event was briefly disrupted at the beginning of Sanders’ speech when vegan protesters carrying “Let Dairy Die” signs started chanting. At least one topless female protester appeared to be led off the stage by security.

Sanders is the latest presidential contender who’s come through California in recent days, many stopping over from neighboring Nevada, where voters caucus on Saturday.

Former South Bend Mayor Pete Buttigieg held a rally in Sacramento and did several fundraisers in the Bay Area on Friday. Sen. Amy Klobuchar was also set to hold a fundraiser in San Francisco on Sunday, but she canceled her appearance after coming down with a cold and called in to the event instead.

President Trump is set to swing through Los Angeles, Riverside County and Bakersfield for fundraisers and other events on Tuesday and Wednesday, including a fundraiser at the Palm Springs-area estate of Oracle chairman Larry Ellison.

Meanwhile, early voting is well under way in California. As of Sunday, more than 862,000 Californians had already returned their ballots, according to PDI’s tally — roughly 5 percent of all the ballots sent out. That’s more than at this point during the last presidential primary: In 2016, just about 582,000 votes had been cast by mail with 16 days to go until election day.

The increase is at least in part due to changes in voting procedures around the state that have increased our use of vote-by-mail, with more than 15 million voters around the state receiving their ballots in the mail. It’s too early to say how overall turnout will compare with the 2016 primary, when Sanders lost California to Hillary Clinton by 7 percent.