The nation’s hard-fought presidential primary contests have brought more than 850,000 new voters onto California’s registration rolls, dwarfing numbers seen in 2008 and 2012, according to figures from the state’s 58 counties.

With the voter registration deadline for the June 7 primary coming up on Monday, those numbers are guaranteed to increase.

“We’re looking at a larger voter growth than ever seen before a primary,” said Paul Mitchell, vice president of Political Data Inc., which compiled the registration data. “It’s double the preprimary growth in 2012.”

Not surprisingly, the newly registered voters are overwhelmingly young people, with 37 percent under 25 and 64 percent 35 or younger. Many of the older voters are probably new citizens or people who have moved to California from other states.

Just under 29 percent of those new voters are Latino, Mitchell said, more than double the percentage in 2012.

Along with those new voters, almost 600,000 other Californians have re-registered since Jan. 1, usually because they changed their name, address or party. Those 1.45 million new or updated registrations as of May 1 could grow to 2 million by the registration deadline.

These numbers don’t show up in the most recent voter registration figures released by the secretary of state, which found there were only 9,420 more registered voters on April 8 than there were on Jan. 5. But the state figures reflect only the net increase, which is the number of new voters minus those who died, moved or were removed from the rolls for other reasons.

Updating lists

With the federal government growing increasingly concerned about election security, “the state has been putting a lot of pressure on counties to clean up their voting lists,” Mitchell said.

In early April, for example, Orange County moved more than 107,000 voters who hadn’t cast ballots in four years to inactive status, which means they wouldn’t appear in the state figures.

While Contra Costa County hasn’t taken any inactive voters off its voter lists this year, 6,471 were removed for other reasons, cutting into the county’s growth of 22,240 new voters. In Santa Clara County, 21,790 new voters were added as of April 8, while 11,351 were removed from the rolls.

But Mitchell’s company has reason to dig deeper into the registration data, since it provides a wide range of voter and demographic information to political consultants across the state, who use it to target voters for campaign flyers and other political activity.

“When you get a campaign mailer in your mailbox, you can blame us,” he said.

Overwhelmingly Democratic

The registration figures are bad news for the state’s Republicans, who already have seen their numbers falling in California. Nearly half the new voters, 48.5 percent, registered as Democrats, compared with the 16.7 percent who registered as Republicans and the 34.8 percent who either declined to state a party preference or registered with a minor party.

The Democratic number is slightly above the current statewide 43.7 percent registration figure, but the percentage of new Republicans is far below the party’s 27.5 percent statewide registration, which could hurt GOP candidates both in the fall and in the future.

While it’s likely that many of the new Democratic voters signed on specifically to cast ballots for either former Secretary of State Hillary Clinton or Vermont Sen. Bernie Sanders in the party’s presidential primary, registration isn’t the same as turnout, said Mark DiCamillo, who runs California’s Field Poll.

“There are a lot of variables involved,” he said. “It’s a tricky business trying to evaluate turnout.”

Party contrasts

Still, with the Clinton vs. Sanders contest continuing to draw headlines, “Democratic turnout is likely to be huge,” DiCamillo said. With businessman Donald Trump virtually guaranteed the GOP nomination, Republican turnout “is not likely to be as high, since (the nomination) is not in play.”

The differing primary election rules of the Democrats and the Republicans are likely to add a note of confusion as Monday’s registration deadline approaches.

While the Democrats allow voters who register as “No Party Preference” to cast ballots in their presidential primary, it’s not automatic, said John Arntz, San Francisco’s election chief.

Independent voters who want to be mailed the Democratic ballot, “have to send us something,” he said. “They can’t just make a phone call to get it done.”

City election officials have already received a few complaints from no-party-preference voters who want to know why the presidential primary wasn’t on the mailed ballot they received, Arntz said.

“That number is going to increase as we get closer to election day, as people become more engaged,” he said.

San Francisco already has sent out more than 20,000 crossover ballots for the Democratic primary, with many more likely to come, Arntz added.

The good news is that there is no time limit for signing up for a Democratic ballot. Independent voters can ask for that ballot even at their precinct on election day and those who get their ballots by mail can exchange them in advance at the county election office in City Hall or at the precinct.

It’s a different story on the GOP side, where only Republicans can vote in the Republican primary. That means anyone, including independents, who wants to vote for or against Trump has to change their party registration by midnight Monday.

It’s even simpler for anyone who doesn’t get their registration done by the Monday deadline. They won’t be allowed to vote on June 7.

John Wildermuth is a San Francisco Chronicle staff writer. Email: jwildermuth@sfchronicle.com Twitter: jfwildermuth