By Eric Bradner, CNN

(CNN) -- For Democrats hoping to turn President Donald Trump's unpopularity into an opportunity to take control of the House next year, this Orange County community is ground zero.

Their target here -- Rep. Ed Royce -- is like several other established Republican incumbents in California who progressives hope can be toppled in a wave of anti-Trump backlash. In their bid to win the 24 seats necessary to take control of the House, Democrats are focusing on California -- and particularly a swath of seven Republican-held seats clustered in or near Orange County.

The reason those seats are being targeted: 2016 Democratic presidential nominee Hillary Clinton bested Trump in all seven districts in the 2016 election. Clinton was the first Democrat since Franklin D. Roosevelt to carry Orange County, and her victory there reflected the rapid demographic changes in the increasingly young, diverse and educated region.

Royce hadn't planned a town hall meeting during the August recess -- so Indivisible, Planned Parenthood and other groups scheduled one without him. Democratic Rep. Linda Sanchez, who represents a nearby Southern California district, attended and fielded questions for hours instead.

Meanwhile, five Democrats who are running against Royce mingled with the crowd of hundreds.

"There's just a lot of built-up anger," said Gil Cisneros, who won a $266 million lottery prize in 2010 and is now challenging Royce. "We have to find a way to keep that going."

"In some ways, this is an opportunity to really bring new people into the fold," said Sam Jammal, a former chief of staff to Rep. Tony Cardenas, an Obama administration Commerce Department official who is also running in the district.

The event underscored the opportunities and challenges confronting Democrats headed into the 2018 midterm elections: The party's base is energized enough to draw a massive crowd on a Thursday night 15 months away from the election. But it has also attracted candidates in droves, which means a long, expensive primary season before a Democrat can fully focus on Royce, who already has $3.1 million in the bank.

Royce and Reps. Dana Rohrabacher and Mimi Walters' districts are located in Orange County. Rep. Darrell Issa's district extends from its south toward San Diego.

Rep. Steve Knight represents the suburbs north of Los Angeles. And Reps. David Valadao and Jeff Denham represent California's Central Valley, not as near to Orange County as others but areas with similar demographics and recent election results that have led Democrats to believe their seats are also within reach.

The Democratic Congressional Campaign Committee for the first time has moved its Western regional office to Irvine, California -- a decision that reflects the group's early commitment to the region. The House Democrats' campaign arm has six staffers working in that office and six more organizers in target districts across the state.

Republicans are moving more slowly, but emphasizing the jam-packed fields of challengers and underscoring the rifts those primary fights could expose within the Democratic Party.

"While Republicans are focused on their districts, Democrats are tearing one another apart in brutal, expensive races to the left," said Jack Pandol, a spokesman for the National Republican Congressional Committee. "Thus far, Democrats' big promises in California have won them nothing except more friendly fire that they are struggling -- and failing -- to control."

An influx of outside spending is also already reaching the districts of Royce, Valadao, Knight and Denham.

The Congressional Leadership Fund, a Republican super PAC aligned with House Speaker Paul Ryan, has opened field offices in each of the four districts, and could open offices in more competitive California districts in the coming months.

The affiliated group American Action Network plans to launch television advertising in the districts of Valadao, Knight and Denham at the end of this week that will give the three Republicans air cover as the party starts a tax reform push, touting popular planks of such an effort. Already, it has launched both English- and Spanish-language radio ads in those districts.

Its organizers are also playing up individual House Republicans' district-focused strengths, such as Denham's work on water issues.

"California is really a state that we know that Democrats are going to be targeting, so we need to start early and be out there talking to voters about what these members are doing back in DC on their behalf," said Ruth Guerra, a CLF spokeswoman.

On the left, billionaire environmentalist Tom Steyer's group NextGen America is focusing its early 2018 efforts on Royce, Valadao, Knight and Denham -- "four districts where young people obviously can be the swing," Steyer said.

He said his group will be on more than 200 college campuses in eight states in the 2018 cycle, organizing young people who will be part of a grassroots program that emphasizes face-to-face conversations and texts with potential voters. In the 2016 election cycle, Steyer said, young people were 23% more likely to vote if they'd spoken with NextGen organizers than if they had not.

That's particularly important in California, where the sky-high costs of advertising in the Los Angeles media market could keep Democrats -- especially those who face competitive primaries -- from keeping up with their Republican foes on the airwaves.

In addition to geography, the House Republicans whom Democrats are targeting have a key vote in common: They each supported the House's bill to repeal President Barack Obama's Affordable Care Act and replace it with legislation crafted by GOP leadership and supported by Trump.

In Rohrabacher's district, Hans Keirstead, a pioneer in stem cell research who sold his business for more than $100 million in 2014 and later bought it back, has given Democrats a top recruit with a science and business background in a district where Republican registration outpaces Democrats.

Keirstead, a 50-year-old Canadian immigrant, said he plans to offer himself as an expert voice on health care with decades of experience in scientific research and pharmaceutical trials and approval -- something he said Congress needs.

"The breadth and depth in health care experience is lacking" on Capitol Hill, he said, arguing that Republicans and Democrats "both want experts there -- and they both want change."

He faulted Trump's administration for slashing spending on Obamacare digital and television outreach and narrowing the enrollment period. "That had a direct effect of increasing premiums," he said.

Then, he said, Trump's threats to end government payments to health insurers caused insurers to "hedge" and increase premiums again.

"Here, months later, we say, 'oh, look at the rising premiums, continually rising.' It was just cresting to decrease," Keirstead said. "That crippling of Obamacare for nothing more than political gain really disappointed me. I just thought, putting politics in front of peoples' health -- it's just so disgusting."

During a tour of his Aivita Biomedical offices in Irvine, Keirstead also lambasted Trump's administration for proposing a budget that would slash the National Institutes of Health's funding for scientific research.

"We're cutting off the future of medicine and health care," he said.

Royce and Rohrabacher's offices did not respond to CNN interview requests.

California Republicans say they are optimistic that their incumbents' fundraising advantage and their survival in 2016 will position them well in the midterms -- when the party expects a gubernatorial race could help draw some additional Republican votes.

"The fact of the matter is, in a presidential election with near-record turnout, these Republicans won rather handily, even though Hillary Clinton won their districts. They will more than likely outperform their margin of victory in 2018," said Jim Brulte, the California Republican Party chairman.

"We are aware that Nancy Pelosi wants to be speaker. We are aware that she believes her return to the speakership runs through some of these seven California districts," Brulte said. "We are confident we will hold them -- but we don't take anything for granted."