This is from a story on stateline.org, “Independent Voters May Be Vexed At Polls”:

In delegate-rich California, it was state political parties – not state lawmakers – that set the rules for how “decline-to-state,” or independent, voters can participate.

“This is the first time such a system is being used in California,” said Conny McCormack, former registrar of voters and the county clerk for Los Angeles County. She said she feared some voters might be confused by the arrangement. Nearly 3 million voters, or 19 percent of California’s electorate, are registered as independents, she said. At stake are 441 delegates for the Democrats and 173 for the GOP.

California decided in 2000 that “unaffiliated” or “decline-to-state” voters could participate in primary elections if the parties allowed it. While the California Democratic Party opted to let “declare-to-state” voters participate in its primary, state GOP leaders in a controversial decision decided to bar independents from their presidential primary.

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Independent California voters must be careful to ask for a Democratic ballot. Otherwise they automatically will be given a nonpartisan slate, allowing them to vote only for nonpartisan offices and ballot measures.