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SACRAMENTO — Monday is the last day to register to vote or update your party affiliation to vote in the California primary. However,  independent voters must take another step if they’re looking to cast a ballot for president.

When FOX40 visited Bernie Sanders’ campaign headquarters in Sacramento, they were in the last hours of the statewide push to register as many potential voters as possible. For people registered as Democrats, the process is straightforward, but for no party preference voters, also known as independents, it’s tricky.

“We see a lot of people who have got their vote-by-mail ballot and they don’t have Bernie on it because they’re no party preference,” said David Pfau, a Sanders volunteer.

To vote in a presidential primary in California, a voter’s registration must match the party primary they are voting in. So to vote for in the Republican primary you must be a registered Republican. Three parties allow an exception. American Independent, Democrat and Libertarian parties also allow independents to vote in their primaries. But independents must not only register as “no party preference voters,” but must also request a presidential ballot.

“It’s caused mass confusion, especially if not only among non-preference voters,” said Bill Simpich, the Bay Area civil rights attorney who filed the suit.

The lawsuit alleges California’s presidential primary rules relating to independent voters may keep thousands of voters from being able to cast a presidential ballot June 7. The suit claims counties are inconsistent in how they educate independent voters about their options.

The plaintiffs include a Bay Area Bernie Sanders support group called the Voting Rights Defense Project, the American Independence Party –which is a conservative party — and two voters. The defendants are two Bay Area elections officials and Secretary of State Alex Padilla.

“That’s what we’ve been trying for months now to keep putting information out there. We’ve put press releases, information, even social media to let voters know what the rules are,” Padilla said.

“We’re doing anything and everything that we can and so has every county, each county does it a little differently, but each county is required to alert their no party preference voters what the rules are,” Padilla said.

Padilla said he has not seen the lawsuit, but he says his office has put out information online, through press releases, and on social media about voting procedures. He thinks the plaintiff’s call to extend the registration deadline until June 7, Primary Election Day, is unrealistic.

“That’s just not doable, those deadlines are set in state law, and for good reasons, counties need to have that final list and have that rosters to go to every polling place to conduct our election with integrity,” Padilla said.

Attorneys will appear in court Monday regarding the suit, but Padilla expects the May 23 deadline to stand. Voters have until Monday at midnight to register online or until election offices close Monday to register in person. Monday May 23 is also the deadline for independent voters voting by mail to request an American Independent, Democrat or Libertarian presidential ballot.