This year, the richest and most populous US state will be voting on Super Tuesday. Here’s what you need to know about the California primary.

Why does the California primary matter?

In past elections, primary candidates generally skipped over California because the state has been reliably Democratic for quite some time and because its primary fell late in the election cycle.

This year, the primary has been moved up to Super Tuesday, incentivizing candidates to spend more time in California and make a bid for the huge number of delegates to win in the state. California will assign more than 20% of the delegates – or partisan electors – needed by candidates to win the Democratic nomination. A strong showing here would give a candidate the momentum needed to more or less secure victory.

“California is also important because of its diversity,” said Matt Barreto, a political scientist and co-founder of the national polling and research firm Latino Decisions in California. “Whoever wins here send a message that they can win black, Latino, Asian American and white voters.” In California, Latino voters could end up determining how a third of the delegates are allocated, Asian American voters could determine a fifth, and black voters could determine a 10th.

How does the California primary work?

California will send a total of 495 delegates to the Democratic national convention. Most of them, 272, will represent a California congressional district. Candidates who earn at least 15% of the vote in a district are awarded a proportion of that district’s delegates. An additional 144 delegates are awarded to candidates who win 15% or more of primary votes statewide.

An additional 79 unpledged so-called superdelegates will round out the total to 495.

Who’s favored?

The biggest challenge for candidates in California is that the state is simply huge.

“It’s such a big, expensive state to campaign in,” said Eric Schickler, the co-director of the Institute of Governmental Studies at the University of California, Berkeley. “California has tended to be friendly to candidates who are already well-known, or have the means to become well-known.”

In 2008, Hillary Clinton edged out Barack Obama largely because she had more name recognition, Schickler explained. This year, Bernie Sanders, who ran a presidential campaign in 2016, Joe Biden, who was the vice-president, and Mike Bloomberg, whose vast funds have allowed him to blast the state with advertisements, have a slight advantage.

Most recent polls have found Bernie Sanders in the lead. A Monmouth poll found that nearly a quarter of likely Democratic primary voters backed him, and a Public Policy Institute of California survey found the Vermont senator had 32% of the likely Democratic vote statewide.

Sanders has invested heavily in California – building up a network of staff and volunteers to spread his message. His campaign has by far held the most public events in the state – though Pete Buttigieg and Joe Biden have held far more fundraisers, according to an ongoing count maintained by the Sacramento Bee. Across the country, Sanders also has the most support from Latino voters, who make up 35% of the adult population in California.

Warren, who initially topped polls in California, has been falling back since late last year. She, along with Joe Biden, Michael Bloomberg and Pete Buttigieg, were all clustered just under 15% support in the PPIC poll. If the trend holds, Sanders could win the lion’s share of California’s delegates.

Who can vote?

Voters who are registered as members of the Democratic party are automatically eligible to vote. Those who marked “No party preferred” when they registered to vote can also request a Democratic primary ballot, via mail or in person on election day. However, independent voters have to register as Republicans in order to vote in that party’s primaries.

A new measure allows Californians to conditionally register to vote on election day and during the two-week period before it, which could “significantly increase” turnout, according to the measure’s senate floor analysis. There are 5.4 million unaffiliated voters in the state, and the new rules require less time and paperwork for those who want to update political party preference or address information at the last minute.

When can we expect the results?

“If the race really tightens up, it could take some time,” said Schickler.

Voting by mail began on 3 February, the same day as the Iowa caucuses, and about 16m ballots have been mailed out. Mail-in ballots that are returned as late as the Friday after the elections are counted, as long as they are postmarked by 3 March.

Voters have been especially slow to return their ballots this year. The race is “totally up for grabs, so people are holding on to their ballots till the last minute”, said Ace Smith, a Democratic presidential consultant. Election officials have a month to verify the ballots, which means if it’s a close race, the outcomes reported on election night could change weeks later.

Potentially complicating matters is a 2016 law designed to make it easier for California’s 20.5 million voters to participate in elections. As a result of the law, some California counties are testing out new electronic voting machines, forgoing traditional polling locations in favor of larger “voting centers”, and expanding early voting and vote-by-mail options. The trouble is, many Californians don’t know about these changes, which could cause chaos on election day and potentially delay the vote count.