For Bernie Sanders to defeat Hillary Clinton in California’s June 7th Democratic presidential primary, he’ll likely need strong support from independents. But new data suggest the Sanders campaign faces a big hurdle.

Take 67-year-old Paul Taylor, your typical Bernie Sanders supporter: passionate about income inequality, disgusted with the Democratic Party, and a registered independent (or what California calls “no party preference”).

There’s just one problem: Taylor’s vote-by-mail ballot does not have a single presidential candidate on it.

“I was disappointed as hell,“ said Taylor, who came from nearby Martinez to watch Sanders speak at a campaign rally in Vallejo Wednesday night. “I really did not realize that I was not going to have an option to vote for Bernie in the primary.”

Actually, “no party preference” voters can vote in the Democratic primary – if they specifically request a Democratic ballot. That’s easy enough if you vote at a polling place. But if you vote by mail, like Paul Taylor does, it’s a lot harder: You have to submit a form to your county elections office.

Taylor didn’t know that – “I expected to get a Democratic ballot” – but here he was anyway, waiting for Sanders in Vallejo.

If this year’s earlier Democratic primary contests are any guide, Sanders will not be able to win California without a lot more voters like Taylor. Yet according to the consulting firm Political Data, Inc., less than 15 percent of the 2.1 million independent Californians registered to vote by mail have requested a Democratic ballot so far.“

A lot of the time, the education just hasn’t been done,” says  Elena Salisbury, a regional field director for the Sanders campaign. “And so we are working overtime to make sure that we contact as many of these people as possible so that they have a chance to vote for the candidate that they want to support.”

For voters like Paul Taylor, all is not lost. They have until May 30th to request a Democratic primary ballot from their county elections office. Or they can bring their mail ballot to their polling place on Election Day and request a traditional ballot instead.

The question is whether enough of them will do so to give Sanders a shot in California against Hillary Clinton.