A big win in the state could help the senator stay afloat. But nothing is certain as counting continues: ‘It’s like a black box’

Bernie Sanders was declared the winner in California just moments after polls closed but his final haul of delegates could remain uncertain for days, or even weeks, as election officials tally the millions of provisional votes and mail-in ballots.

And while triumph in the Golden State will help Sanders remain competitive in what has become essentially a two-person race between him and Joe Biden, the eventual delegate math could indicate a decidedly shakier path to the nomination than the Vermont senator expected last week.

Voting on Super Tuesday in Los Angeles county, the state’s most populous, was marked by chaos after a $300m new voting system resulted in hours-long lines that kept people waiting even as midnight approached. Meanwhile, nearly half the state’s ballots still remain uncounted, estimates Paul Mitchell, with the campaign research firm Political Data Inc, which tracks ballots as they are returned.

“It’s like there’s this black box of how many ballots are at the post office right now,” said Mitchell. So far, about 5.3m ballots have been counted, according to Mitchell, just over 50% of the 10m he expects have been cast in this election. “The x-factor really is: do we see some big surging coming with the uncounted ballot results?” he said. “It’s just a huge unknown.”

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Many voters dropped off or posted ballots at the last minute, and election officials are required to count mail-ins that arrive as late as Friday, as long as they are postmarked by 3 March.

After all the volatility, withdrawals and endorsements leading up Super Tuesday, many California voters said they felt torn as they headed to the polls – and even after they’d voted.

Stephenie Smith, 31, told the Guardian she had made a last-minute decision to vote for Warren. “After a lot of conversations, I was kind of leaning toward Bernie,” she said. “I had such mixed feelings.” But on election night, she was worried she’d made the wrong choice. “I like Warren slightly better, but I didn’t want to have split the progressive vote,” she said.

Facebook Twitter Pinterest Joe Biden speaks on Wednesday at the W Hotel in West Beverly Hills, Los Angeles. Photograph: David Swanson/EPA

Voters who had their hearts set on Pete Buttigieg, Amy Klobuchar, or other candidates who dropped out before the election were left scrambling as well. Andrew Byrnes, 47, said he settled on Joe Biden, “because I think he’s the most likely to defeat Trump”, he said. “I was torn between him and Warren, but it doesn’t seem like she has a chance of winning.”

Traditionally, the demographics of late voters tend to resemble the types who would favor Sanders, Mitchell said. In California, the Sanders campaign’s early outreach to a dedicated coalition of young, liberal and Latino voters appeared to dull the impact of Biden’s momentum. Exit polls across the country, however, suggested that late deciders tended to side with Biden.

As more votes are counted, support for Mike Bloomberg – who is currently in third place – is likely to wane, Mitchell said.

Whatever the final numbers, Sanders, who has long dominated in state polls, is sure to retain his strong lead in a state where his progressive politics resonate with a Democratic electorate that skews left, said Erick Schickler, the co-director of the Institute of Governmental Studies at the University of California, Berkeley.

Progressive voters have also helped boost down-ballot candidates, including Shahid Buttar – the San Francisco activist and attorney challenging the House speaker, Nancy Pelosi, for her congressional seat. He earned just over 12% support compared with her 73%, but it was enough to put him in second place, with a shot at challenging the 33-year incumbent in November.

At an election night watch party in San Francisco’s Mission district, Sanders supporters cheered and clapped as bar TVs blared news of the lefty candidate’s resounding victory.

Play Video 1:24 Biden triumphs, Sanders hits back and Warren fights on: how Super Tuesday unfolded – video report

Kyle Schmolze, 30, said he felt “better about progressive politics than I’ve ever felt in my life”. Sanders’ sweep in California was heartening. “It feels like we can actually win,” he said.

But his candidate’s losses in the east and south worried Schmolze. “I would feel safer if Bernie had dominated – which is what I had expected,” he said. “I’m feeling all the emotions.”