Fresh-faced Democrats are packing U.S. House races nationwide, inspired to run in the Trump era with hopes of seizing long-held Republican seats in a much-anticipated "blue wave."

But in California, Democrats will find out in June if all that enthusiasm has a downside.

With an open primary that sends the two highest vote-getters to the November general election regardless of affiliation, party leaders fret the crowded fields in several races could splinter the vote enough to lock Democrats out. That fear is most acute in three Southern California districts that are top party targets with multiple Republicans and Democrats running.

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Failing to field a candidate in a California race could have national implications. The party needs to flip at least 24 Republican seats, and the cluster of California districts could prove a critical piece of the puzzle.

The party's national campaign arm joined the fray last week, committing resources to Navy veteran Gil Cisneros in the 39th District, where Republican Rep. Ed Royce is retiring. By putting Cisneros on the "Red to Blue" list, the Democratic Congressional Campaign Committee is signaling to donors that he is the most viable candidate among the six Democrats seeking the seat against seven Republicans.

Meanwhile, incumbent Republican Rep. Dana Rohrabacher, faces four party challengers and six Democrats in the 48th District. Four Democrats and eight Republicans are running to replace Republican Rep. Darrell Issa, who is retiring in the 49th District.

"This is a free for all. This is the Wild West," Jessica Hayes, chair of the San Diego County Democratic Party, said of the 49th District race.

The three districts are among the seven in California targeted by Democrats where voters chose Democrat Hillary Clinton in the 2016 presidential contest but sent Republicans back to Congress. In three other targeted districts, at least four Democrats are running.

Washington state also has a top-two primary system, but Democrats don't fear a shut-out there because no Republican incumbents face serious intra-party challengers. For example, in the 8th District east of Seattle, a handful of Democrats are squaring off against one Republican to replace a retiring GOP incumbent.

The scenario is different in California, where multiple Republicans are running. Although Republicans also have large fields, a few of the candidates in each race are well-known to voters, whereas the Democratic competitors are mostly political novices. In Rohrabacher's district, for example, fellow Republican Scott Baugh is considered one of the most credible challengers because he's served as an assemblyman and chairman of the Orange County GOP.

Democratic strategist Katie Merrill runs a super political action committee aimed at flipping the California seats. It conducted polls in March showing possible shutouts, a scenario she called "an unintended consequence of increased Democratic enthusiasm."

Annie Wright, who became interested in local politics last year and launched a local Orange County Democratic club, said activists are increasingly asking candidates to justify why they are still in the race at debates and town halls. But most ordinary voters don't know who is running, making it hard to predict what will happen, she said.

"We don't have a very good picture right now of who people would vote for," she said.

Some Democrats have dropped out; Michael Kotick exited the Rohrabacher race in early April, urging Democrats to back Harley Rouda and warning the packed field would only help Rohrabacher.

The California Democratic Party endorsed neuroscientist Hans Keirstead in that race. But activists couldn't settle on a candidate in the Issa and Royce districts, nor in the contest against Rep. Jeff Denham in the Central Valley. He also faces a Republican challenger.

Some Democrats and strategists contend shutout fears are overblown. The top-two primary system began in 2012 and has produced a one-party general in a U.S. House race just once, when Democrats failed to secure a spot in a 2012 contest. In statewide races, a Republican failed to advance a candidate to the 2016 U.S. Senate general election contest, setting up a contest between Democrats Kamala Harris and Loretta Sanchez.

Garry South, an adviser to former Democratic Gov. Gray Davis who advocated for the creation of a top-two primary, downplayed concerns.

"If I had to go into an election cycle with a huge amount of enthusiasm in my base which has produced perhaps a surfeit of candidates, I'll take that risk," he said.

Former Democratic U.S. Sen. Barbara Boxer also runs a political action committee targeting Republicans. She does not plan to weigh in on the primaries and said it's not the party's place to tell eager candidates to step aside. But she urged voters to think carefully when casting their ballots.

"The beauty of the free election is that everybody can decide to run," she said. But "if you're worried this could happen in your district, then think long and hard before you throw away your vote for somebody who might only get 2 or 3 percent."