In summary

“Zombie votes,” or votes for candidates no longer running, could be a factor in California’s Democratic presidential primary.

The first batch of votes counted when California polls close tonight will be the more than 3.7 million ballots already mailed in — and a chunk of those will contain “zombie votes” for president.

Not votes for literal zombies, of course — it’s a term politicos use to describe votes cast for candidates who have pulled the plug on their own campaigns. And three Democratic contenders did just that in the past few days.

Paul Mitchell, with the electoral research firm Political Data Inc., projects that 350,000 to 400,000 Californians have already voted by mail for a candidate who has dropped out. And he suspects that number could grow to more than a half million.

Even so, most voters appeared to be waiting until the last minute to cast their ballots (and perhaps make up their minds). Although more than 16 million Californians received a mail ballot, less than a quarter of those ballots had been returned by the eve of the election.

Because those ballots need only be postmarked by tomorrow and arrive by Friday to be counted, officials are preparing for possible problems and delays of days or weeks before the final outcome in close races.

That’s one reason some advocated voting ahead of time — a strategy campaigns urged as well, as a way to lock in supporters and “win” the optics of the night early on the strength of a vigorous showing in the early vote tabulations.

But many of those who voted early for Pete Buttigieg, Amy Klobuchar, Andrew Yang or Tom Steyer are probably more than a tad regretful now at arguably having wasted their votes.

The state says too bad. Once a vote is cast and mailed in, there is no going back and changing your mind. Michigan has a spoiled ballot rule, which allows voters to get a new ballot if the candidate they chose dropped out. California has no such Plan B option.

And what implications might this have for candidates still in the running? Votes for Democratic candidates who quit can affect how many delegates the remaining candidates will be allotted. Party rules dictate that a candidate must earn 15% of the statewide vote to get any statewide delegates, and also that a candidate must earn 15% of the vote in any of the state’s congressional districts in order to receive any delegates apportioned at the district level.

That means “zombie votes” could cause other candidates to miss the 15% threshold.

Mitchell used Michael Bloomberg in a hypothetical. If Bloomberg hypothetically received 13.8% of the vote, but candidates no longer running also receive a percentage of the vote, then “them not being in the race could easily have gotten Bloomberg over the top of 15%.”

“But since those votes are still counted in that delegate math,” he said, “Bloomberg could theoretically stay under 15% just because of all the other candidates that have sucked up votes. And those votes count in the calculations.”

It’s also very possible that a candidate who has dropped out will nonetheless exceed the threshold enough to win some delegates in certain congressional districts. Those delegates would be chosen by the ex-candidate and expected to vote for him or her on the first ballot at the Democratic National Convention.

As for the anticipated flood of same-day voting, either in person or by mail, it’s difficult to say precisely which candidate stands to benefit most. Joe Biden, for instance, has been buoyed by a robust victory in the South Carolina primary and a stream of quitting candidates — Klobuchar, Buttigieg and Beto O’Rourke among them — who endorsed him. Californians who didn’t vote early may be swayed by all that good news for Biden.

Conversely, the former vice president lost that momentum propulsion among all the Californians have already voted by mail at a time when he trailed other contenders.

Heading into the final hours of the California primary, the latest available public polls still indicated Bernie Sanders with a significant lead.

Polls close at 8 p.m. For more on the candidates, check out the CalMatters voter guide.

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Elizabeth is a general assignment reporter for CalMatters. She graduated from Chico State with a bachelor's degree in journalism. While in college at Chico, Elizabeth did internships with the local NPR...