SAN FRANCISCO (AP) — Officials in California are bracing for long lines and urging patience as voters cast ballots on “Super Tuesday” in what could be record turnout for a presidential primary election.

There was a steady stream of voters at San Francisco’s City Hall early in the afternoon but officials said they expected crowds to grow later in the day.

A fraction of the 20.7 million registered voters in the heavily Democratic state already returned mail-in ballots in early voting, which started last month.

Officials expected many more mail-in ballots to arrive Tuesday because many voters waited to see results from other primaries, like last weekend’s South Carolina contest, before mailing in their ballots.

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“We got 20,000 ballots in the mail today and many people are showing up at the polls and dropping off their mail-in ballots,” said John Arntz, director of San Francisco’s Department of Elections.

In Santa Clara County in the San Francisco Bay Area, turnout at voting centers appeared light but the area’s voters usually show up after work, said Evelyn Mendez from the Santa Clara County Registrar’s Office.

She said the office had received about a quarter of the mail-in ballots that were issued.

“People tell us they were waiting to the last minute to see if candidates dropped out,” Mendez said.

Enthusiasm is high among Democrats eager to elect a candidate they hope can oust President Donald Trump in the fall, and California moved up its primary from June to March so voters could weigh in earlier.

The state has been blanketed by continuous advertising from billionaires Mike Bloomberg and Tom Steyer, who dropped out of the race Saturday after a third-place finish in South Carolina. Sens. Bernie Sanders and Elizabeth Warren and former Vice President Joe Biden are still running for California’s 415 pledged Democratic party delegates.

San Francisco resident Tara Ramroop, 38, dropped off her ballot in a box outside of San Francisco City Hall and said she voted for Warren because “her message, her style, her wit resonated with me.”

“I think she is the smartest candidate to lead us, hopefully, in the post Trump era,” said Ramroop, who works in marketing, adding that she would vote for whichever Democratic candidate gets the nomination,

California’s primary also coincides with a number of changes aimed at expanding voter participation. Those changes may end up confusing voters or contributing to longer lines.

In casting his ballot at an early voting center last month, Secretary of State Alex Padilla said that election day wait times may be longer than normal given the number of people registered to vote. Voters are also weighing in on congressional races, state legislative seats and a statewide school bond.

“I’m expecting we’re going to see an avalanche of ballots on election day, and it’s going to take a while to figure out the results,” said Kim Alexander, president of the nonpartisan California Voter Foundation.

New this year, Californians will be able to register to vote through 8 p.m. Tuesday at any location where ballots are accepted, which could tie up lines as people fill out paperwork. Results may be delayed because provisional ballots take longer to count.

Also, 15 counties representing more than half the state’s voters have replaced traditional neighborhood polling places with a smaller number of multi-purpose vote centers where people can register, vote and take care of other elections business.

The new centers are designed to make voting more convenient, but may confuse people who are accustomed to visiting their local polling place.

Los Angeles County reported several hiccups when it rolled out its vote centers last month. A handful opened late or not at all because equipment didn’t arrive in time or workers didn’t have correct information to start new touch-screen ballot markers.

Sacramento County will be tripling staff at a voting center at Sacramento State University that saw huge lines in November 2018 as students raced to register at the last minute. Everyone in line by 8 p.m. got to vote, even if that was hours later, spokeswoman Janna Haynes said.

Elections officials have been encouraging people to vote early, in case of problems and to avoid election day mayhem. But voters like to hang on to their ballots, perhaps more so for an election with a wide-open presidential primary.

Voting advocates hope the waits won’t discourage voters.

“Overall, California has suffered from long lines a lot less than other states,” said Jonathan Stein, head of the voting rights program at Asian Americans Advancing Justice-Asian Law Caucus.

“The long line is unfortunate but ultimately, it’s a product of California trying to do the right thing by voters,” he said.

Associated Press writers Olga R. Rodriguez and Jocelyn Gecker contributed to this report.