More than 5 million people have requested mail-in ballots in California, where there are 15.7 million registered voters. Election officials in the most populated and delegate-rich state in the country have said results may not be available until Wednesday or later.
As much as 25 percent of the overall vote may go uncounted Tuesday night, officials said. A major cause of expected delays is late-arriving mail-in ballots, which will be counted only after precinct votes are tallied. Polls close at 8 p.m. PST.
Another element is the state's recent switch from electronic voting machines to paper ballots. Four of California's most populous counties -- Riverside, San Bernardino, San Diego and Santa Clara -- must count votes at centralized locations because there aren't enough optical scanners for every precinct. Los Angeles and Sacramento will also haul their paper ballots to a single location, where they will be tallied electronically.
"We're working as late as we can to get all of them counted," said San Bernardino registrar Kathi Payne.