A lot of voters with no party preference who were hoping to cast ballots in California's June 7 Democratic presidential primary could be in for an unwelcome surprise if they don't act fast.

Only registered Republicans can vote in the GOP’s presidential primary. But California's Democratic primary is open to no party preference voters and they are free to participate in the primary -- that is, if they request a Democratic ballot. Data analyst Paul Mitchell says most independent voters haven't done that, even though they were mailed a notecard asking if they wanted a specific party ballot. He says 85 percent of ballots sent to no party preference voters statewide did not include a presidential option.

The primary is just days away, but Mitchell says there's still time for independent voters to get a Democratic ballot.

Two Ways to Get a Democratic Ballot

"One avenue is to find that notecard or to download it from the county registrar website," he says. "Hand-sign it, fill it out, mail it in. And if the county registrar receives it before May 31, which is coming up soon, they can get a new ballot mailed to them. The ballot they are currently holding would become invalidated."

Mitchell says voters can also go to their polling place on Election Day and swap out their ballot for one with the presidential contenders.

To get a sense of how many voters might be affected, there are a few voting-related numbers to consider. One is the growing number of independent voters in the state, now nearly 24 percent of all registered voters, or more than 4 million people. Mitchell says his research shows about half of those voters want to take part in the Democratic primary. The other factor is the growing number of people who vote by mail. They accounted for more than 60 percent of voters in the 2014 general election.

Mitchell says the situation is further complicated because the need to request a specific ballot happens only in presidential election years.

"This is a one-every-four-year occurrence," he says. "Now being told, 'Oh, you were supposed to have mailed in a notecard,' is frustrating a lot of voters."

The situation has added up to some sleepless nights for Ben Tulchin, the pollster for the Bernie Sanders campaign. He says the ballot situation could cost Sanders a couple hundred thousand votes.

“Any time there are barriers to voting, we all know from years and years of experience, some people won’t be able to overcome those barriers and vote in the way they want,” he says.

Tulchin says the campaign is still confident it can win California. A recent poll shows Sanders has closed the gap with former Secretary of State Hillary Clinton.