A fraction of the 20.7 million registered voters in the heavily Democratic state has already returned ballots in early voting, which started last month. Officials expect the bulk of ballots to be cast Tuesday.
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.
But the 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.
SUPER TUESDAY: 2020 Primary Election Voter's Guide
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.
Officials at the Fresno County Elections Office have been busy leading up to election day. The office is also expecting a busy turnout for Super Tuesday.
The county clerk, Brandi Orth, said as of Monday, 100,396 ballots were returned either by mail or in drop boxes.
About 1,500 voters in Fresno County were notified by mail because their returned ballots either were not signed or the signatures were different from the ones on file.
Orth said voters have until today to postmark their ballots and signature verification letters.
"If you get one of those letters, then act swiftly and return the letter to a vote center or to my office," Orth said.
Also, 15 counties in the state, 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.
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.
In Fresno County, so far, less than 25% of ballots have been received county-wide.
During the last primary two years ago, the county clerk's office received more than 50% of ballots returned.
Fresno County election officials expect to see a bigger voter turnout in November.
The Associated Press contributed to this report.