KQED:

Presidential challengers hoping to glide to victory through California’s newly relevant primary, a heads-up: Your electoral fate may hinge on convincing enough left-leaning millennials to send postcards over the holiday season.

Welcome to the quirk-filled world of California election law. Here, voters without a registered political party can participate in the Democratic Party’s “open” presidential primary—but only if they ask for the right ballot.

Those who vote the old-fashioned way, in person at the polls, can simply request their presidential ballot of choice on the spot. But for those who vote by mail (now a majority of the state’s electorate), that request takes a remarkably analog form: a postcard signed and sent to the county registrar of voters.

If voters skip that step, the section of their ballot reserved for presidential candidates will be blank.