A dramatic change planned for California elections next year is morphing into a partisan battle over how the state’s ballots should be cast.

When Gov. Jerry Brown signed SB450 in September, it was billed as a new way to boost California’s falling election turnout. Mailing a ballot to every voter in participating counties and replacing the traditional neighborhood polling places with a relative handful of community voting centers would cut costs and make it easier to cast a ballot.

“This landmark law will provide voters more options for when, where and how they cast a ballot,” Secretary of State Alex Padilla, who sponsored what has been dubbed the California Voter’s Choice Act, said in a statement at the time. The bill, he said, “will increase civic participation and make our democracy stronger.”

But Padilla was far less jolly last month after Orange County supervisors, worried about what they said was the potential for abuse, unanimously refused to sign on to his plan, dismissing it without discussion.

“I have real concerns about voter fraud,” Supervisor Todd Spitzer, who made the motion to reject Padilla’s plan, told the Orange County Register.

In an unusually blunt letter to the supervisors, Padilla urged them to reconsider, arguing that the board’s refusal “to allow a public discussion of these proven reforms is particularly concerning.”

The secretary, a former Democratic legislator, slammed the all-Republican board, arguing that its decision “was driven less by the interests of the people of Orange County and more by political considerations.”

While Orange County had a right to reject the voluntary voting plan, Padilla was outraged by the way it was done.

“This is absolutely a concern, which is why I spoke out right away,” Padilla said in an interview. “If there’s a discussion, I’m good to go with it. But it has to be done the right way and not with the false issue of voter fraud.”

While Padilla talks about the good-government aspects of his voting plan, in California bringing new voters to the polls generally means bringing new Democratic voters to the polls. Young people, low-income residents and minorities make up a large chunk of the state’s nonvoters. They’re also demographic groups likely to register as Democrats. Making it easier for them to vote isn’t likely to help the GOP’s already-shrinking numbers in California.

That concern hasn’t been lost on Republicans, even before President Trump began complaining, without providing any evidence, that the only reason he lost the popular vote in November was because of millions of illegal votes cast in California and a couple of other states.

“We have a philosophical divide with our Republican colleagues on vote-by-mail,” said Assemblyman Kevin Mullin, a San Mateo Democrat who helped shepherd the bill through. “They talk about the increased likelihood of voter fraud, even though there’s little evidence that it’s a problem and no evidence that it’s linked to vote-by-mail.”

But Republicans and their allies also may be reacting to what they see as a full-court press by Padilla and other Democrats to expand the voter rolls without considering the consequences.

In the past couple of years, bills have been passed that automatically register people as voters when they receive or renew their drivers’ licenses, allow people to register to vote on election day, count mail ballots that arrive after election day and grant permission for people other than relatives to collect and turn in vote-by-mail ballots.

“We’re not trying to keep people from voting, but we’re worried that there aren’t enough safeguards,” said David Wolfe, legislative director of the Howard Jarvis Taxpayers Association, which opposed Padilla’s bill. “We’re concerned about vote fraud.”

It’s not only Republicans who are concerned about the bill.

“Some of the concerns are legitimate,” said Kim Alexander of the California Voter Foundation, a group whose goal is to modernize California elections and make it easier for people to vote in the state.

Part of the problem is that the new rules will take effect in three stages, with 14 counties cleared to move to the new voting system in 2018, most of the rest of the state in 2020 and sprawling Los Angeles County, which contains more than a quarter of the state’s registered voters, expected to join after 2022.

Each of those counties has to set up its own rules for implementing the plan.

“Since this is happening county by county, there’s a real possibility for confusion,” Alexander said, “with one county getting all mail ballots and the one next door having different rules.”

The state also allows counties to opt out of the program, at least in part because if the state required the new system, the state would have to pay for it.

That’s one of the reasons that the refusal of Orange County, one of those 14 counties in the first wave, is so significant.

If the partisan tag the Orange County supervisors gave the voting bill sticks, it could make it tougher for Padilla to spread his program statewide.

“A lot of the smaller, rural counties in the state are run by Republican boards of supervisors,” Alexander said.

In the Bay Area, Napa, Santa Clara and San Mateo counties were supposed to be among that first group of counties. While San Mateo and Napa counties are ready to go for 2018, Santa Clara County will delay until at least 2020, citing the costs and short timeline for the changeover.

San Francisco and the rest of the Bay Area will be watching to see what happens next year.

“For us, it’s going to be a matter of observing and learning from the problems the other counties have,” said John Arntz, San Francisco’s elections chief.

There always are going to be concerns, though, he said.

The new voting system “is going to basically extinguish polling places from our city, and there’s a real attachment in San Francisco to local polling places,” Arntz said.

Even though about two-thirds of the city’s vote come from mailed ballots, as many as 25 percent of them are still hand-carried to the polls.

San Francisco’s unique position as both a city and county also could make a difference in its election rules, Arntz added.

Padilla, though, believes the new voting plan is flexible enough to deal with any local problems.

He has no intention of stepping back from his efforts to make it easier and more convenient for Californians to vote.

“I stand by my policies,” he said. The new voting rules “bring more people who are eligible to the polls, and that’s a good thing.”

John Wildermuth is a San Francisco Chronicle staff writer. Email: jwildermuth@

sfchronicle.com Twitter: @jfwildermuth