SACRAMENTO, Calif. (Reuters) - As Californians prepare to go to the polls on Tuesday to choose candidates in primary elections, the state’s quirky electoral rules and a softening of support for Democrats among suburban voters could disrupt the party’s ambitious plan to wrest as many as 10 seats from Republicans in California.

November’s general election could change the balance of power in the U.S. Congress, where Republicans control both chambers. Democrats need to win 23 seats nationwide to have a majority in the House of Representatives, considered a very real possibility. Most of California’s congressional districts did not go for the Republican Party’s leader, Donald Trump, in the 2016 presidential election.

“It’s the first time in years that California elections have mattered in national politics,” said Raphael Sonenshein, executive director of the Pat Brown Institute for Public Affairs at California State University Los Angeles. “The House races are critically important.”

Voters on Tuesday will also be choosing candidates to run in next November’s election in primaries in New Jersey, Mississippi, Alabama, Iowa, Montana, South Dakota and New Mexico.

The California Republican party’s biggest problem is a continuing decline across the state that began decades ago.

For the first time, the number of registered voters who are Republicans slipped below the number of independents, leaving the party in third place, said Paul Mitchell, vice president of Political Data www.politicaldata.com, which analyzed registration information from the state this week.

But that decline does not give the Democratic Party an easy win on Tuesday. The party has its own challenges. If voters select Democratic candidates who are too progressive for conservative areas of California, it might prove difficult for Democrats to win in November. Yet if more moderate candidates win in the primaries, fired-up progressives may decide not to vote in November, leaving more room for Republicans to win.

"The Democrats have their work cut out for them," said Mark Baldassare, who runs the Public Policy Institute of California www.ppic.org, a polling and public policy think tank. "Some of the exuberance about Democrats taking this over has maybe gotten a little bit ahead of itself."

So many enthusiastic newcomers have tossed their hats in the ring in an effort to unseat vulnerable incumbent Republicans or to claim seats vacated by resigning Republicans that they may simply divide the anti-Trump vote among themselves.

FILE PHOTO: A poll worker places a mail in ballot into a voting box as voters drop off their ballot in the U.S. presidential primary election in San Diego, California, United States June 7, 2016. REUTERS/Mike Blake/File Photo

That dilution of the vote matters more in California because of its so-called jungle primary. It does not hold separate party primaries. Instead, the top two vote-getters advance to the general election regardless of party.

For example, in the 48th Congressional District, which includes affluent coastal towns between Los Angeles and San Diego, 16 candidates will appear on Tuesday’s ballot. Five Republicans, eight Democrats, a Libertarian and an independent have filed papers to challenge Republican Representative Dana Rohrbacher.

One of the Republican challengers, former State Assemblyman Scott Baugh, is considered well known enough to possibly make it into the second position after Rohrabacher, a situation that would lock Democrats out of the race.

Key to the Democrats’ efforts is suburban Orange County south of Los Angeles, where four congressional districts were once the heart of rock-ribbed California Republicanism.

In all four districts, some of which include parts of San Diego and Los Angeles counties, voters in 2016 chose Hillary Clinton over Trump for president, but returned Republicans to Congress. Two representatives, Darrell Issa and Ed Royce, chose not to run for re-election this year. Rohrabacher and Mimi Walters are seeking re-election in competitive races.

Many voters in those districts are affluent and suburban, a group that has been leaning toward choosing Democrats for Congress in November, according to recent Reuters polling. Orange County is more ethnically diverse than in the past, a change that generally leads to more support for Democrats.

But Reuters/Ipsos polling over four weeks in May shows a softening at the national level of support for Democrats for Congress among suburban voters.

And the Public Policy Institute's May survey of likely voters showed that most likely voters in the 10 Republican-held California districts viewed as very close by the Cook Political Report here were supporting Republicans for Congress.

State Republican Party Chairman Jim Brulte said the polls and a strong presence in early voting indicate Republicans in the state are becoming more engaged as the election approaches, possibly alleviating a so-called enthusiasm gap he had feared would keep Republican voters at home.

“Based on what we’re seeing now, my concerns have been allayed,” Brulte said.

Absentee ballots received so far show Republicans punching above their weight, accounting for 34 percent of ballots returned even though they only amount to 26 percent of registered voters, according to Political Data.