SANTA ANA, Calif.—Bernie Sanders volunteer Kate Hallinan can’t count how many times she’s talked to California voters who are surprised or confused when they don’t see presidential candidates on their vote-by-mail ballots.

That is happening because about 5.3 million people in California are registered as having no party preference, which means they are unaffiliated with a political party. And while they can vote in the Democratic presidential primary on Tuesday, they need to request a so-called crossover ballot to do it.

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