When ballots for the March 3 primary start arriving in the mail next week, more than a quarter of California’s registered voters may be shocked to find they’re cut out of the presidential race.

The growing number of voters who register as “no party preference” won’t find a spot on their ballots for the Democratic presidential race unless they have specifically asked for a Democratic ballot.

That’s not going to make them happy, said John Arntz, San Francisco’s elections chief. His office is already gearing up for a flood of angry calls when the first ballots arrive Monday or Tuesday for those who vote by mail.

“We opened our phone bank earlier this month and immediately started getting calls about the presidential primary,” Arntz said. “But we’re adding more people for next week.”

The problem applies only to the presidential race. In every other contest in California, the state uses a “jungle” primary system, where all the candidates regardless of party are listed on a single ballot, everyone votes and the top two finishers move on to the November general election.

In presidential primaries, though, the national parties set the rules. Since no-party-preference voters don’t belong to a party, they don’t have any candidates in the primary.

While Republicans bar anyone not registered as a member of the GOP from their primary, Democrats have opened their presidential primary to all independent voters. But those voters have to ask for that Democratic ballot.

That’s a message election officials across the state have been working to get out for months.

In November, California Secretary of State Alex Padilla put out a guide on the presidential primary rules for voters of all parties, Sam Mahood, a spokesman for Padilla, said in an email. In December, county election officials sent crossover ballot request postcards to independent voters and followed up with emails.

“Since fall of last year, (no party preference) voting rules has been one of the primary focuses of our social media posts,” Mahood said. “We anticipate running paid social media ads to inform voters of these rules as well.”

The new state voter pamphlets, which were sent out last week, also include a page telling people “How to Vote for President.”

The state and local efforts have had an effect. In San Francisco, more than 15,000 of the city’s 154,000 independent voters have already requested the Democratic ballot, said Arntz.

“But there are certainly going to be people who haven’t picked up the information,” he said.

For independents voting at the polls, there’s no problem. They can simply request a Democratic ballot right there. If they want to vote in the GOP primary, they can change their registration to Republican at the polling place, even on election day, Arntz added. The American Independent and Libertarian parties also allow independents to vote in their primaries.

It’s only a bit more complicated for no-party-preference voters who have already received their president-free ballots.

Even if they have those ballots in hand, there’s still plenty of time to request a Democratic ballot and have the original one canceled, Arntz said.

In San Francisco, that can be done online, by going to the voter portal at the elections office website and requesting a crossover ballot, which will be sent in the mail. It’s a similar process for voters in other counties.

“Voters don’t have to surrender their original ballot,” Arntz said, although they have to remember to cast only one of them.

The online option works only until a week before the election, since the city won’t mail out ballots after that.

No-party-preference voters “still can get a Democratic ballot, but they have to do it at a polling place or City Hall,” Arntz said.

Reaching those voters is a big deal in California, which had 5.4 million independent voters as of Oct.1, the most recent state registration report.

That’s more than the entire population of 28 states, including all four of the early-voting states: Iowa, New Hampshire, South Carolina and Nevada.

For the Democrats, that’s a lot of potential voters in a presidential primary contest that polls around the country say is razor-tight. Party officials are hoping that independent voters who vote for a Democratic presidential candidate in March will stick with the Democrats in the November battle against President Trump.

John Wildermuth is a San Francisco Chronicle staff writer. Email: jwildermuth@sfchronicle.com