By Maeve Reston, CNN

(CNN) -- In California, the heart of the progressive resistance to President Donald Trump, it is Democrats, not Republicans, who are most concerned that a swell of liberal energy could blow their shot at the House majority this year.

Since assuming office, Trump has declared war on California, challenging the state's stance on sanctuary cities, off-shore drilling, and fuel efficiency standards and accusing the mayor of Oakland of obstructing justice.

Trump's actions have been met by a surge of progressive activism in the state, and Democrats are hoping the state's June 5 primary will deliver an opening blow at the ballot box that would send a message to Trump and eventually deprive Republicans of their majority in the House of Representatives in November.

But that plan has collided with the reality of the way California's primary works. The top two vote getters will advance to the November contest, regardless of party, and the churning anti-Trump fervor in the Golden state has been a double-edged sword.

In several of key races, a crowded field of strong, well-funded Democrats run the risk of splitting the vote, which could mean two Republicans advancing to a general election — locking Democrats out in districts that would have been competitive in the fall.

Winning the seven GOP-held congressional districts in California won by Hillary Clinton has long been at the center of Democratic plans to notch the 23 House seats they need nationally to capture the majority.

In these final weeks, the unpredictability of those races has forced the Democratic Congressional Campaign Committee, as well as outside groups, to engage in what has amounted to a multi-million-dollar game of whack-a-mole.

Campaign strategists and outside groups have resorted to an elaborate series of chess moves in which they have battered third, fourth and even fifth-tier candidates to drive down voter support for those contenders in the hopes of avoiding a splintering of the vote.

California Republican Party Chairman Jim Brulte pointed to the outside spending by Democrats in the race for retiring Congressman Ed Royce's seat in Orange County as the best evidence of Democratic fears.

"The generic ballot is doing better" for Republicans," Brulte said. "The tax cut is becoming more and more popular." He also noted that Trump's approval rating has ticked up in some of the Republican-held districts.

The fight for Royce's seat was complicated by the fact that the two self-funded Democrats, Andy Thorburn and Gil Cisneros, had pummeled one another with such force that the California Democratic Party had to intervene to broker a truce.

As outside groups poured more than $3 million into California races last week alone, the independent arm of the DCCC spent more than $200,000 against Republican Bob Huff and nearly $142,000 on ads against his GOP rival Shawn Nelson to knock them from contention for second place.

There are similar maneuvers going on in the race for governor where Democratic Lieutenant Governor Gavin Newsom is the clear frontrunner, but Democrat Antonio Villaraigosa, the former Los Angeles mayor, is targeting Republican candidate John Cox, who was endorsed by Trump, as they compete for the second slot.

Some of Newsom's commercials appear to attack Cox, but the underlying message that Cox is a hardline conservative could actually help shore up Cox's support and ensure his spot on the November ballot.

"It just kind of blows your mind, everyone has these moves, but they are all designed to somehow influence who is going to be number two," said Bill Carrick, a veteran Democratic strategist who is running the campaign of US Sen. Dianne Feinstein. "When you put all these candidates—the minor party candidates, no party preference candidates, the Democrats and Republicans—all on the ballot, you end up with a whole lot of names and it's very confusing."

"Nobody is very well known," Carrick said. "We haven't had the big accident yet. But we're going to."

Because of those concerns in the 48th Congressional District represented by Dana Rohrabacher, two female candidates voluntarily withdrew in an effort to improve Democratic chances of winning the seat. Even now, there is still a risk of Democrats getting boxed out because of the fierce contest between candidates Harley Rouda, the DCCC-backed candidate, and Hans Keirstead, a doctor and scientist who developed innovative treatments for late-stage cancers, ALS and spinal cord injuries.

On Tuesday, the House Majority PAC, a Democratic group, put $650,000 behind an ad attacking Rohrabacher's leading Republican challenger, former state Assembly member Scott Baugh.

Still, Democratic strategists remain confident that California holds the key to a majority in November. Winning some of the seven Republican seats in Clinton districts would be a symbolic as well as strategic victory. Those seats represent the last bastion of GOP power in a state where no Republican holds statewide office.

A surge of activism

The 2016 election produced a new crop of Democratic activists in California who stayed engaged over the past two years through groups like Indivisible.

As Trump's approval ratings have ticked upward nationally, Republicans have grown more hopeful about maintaining their edge in November, even here in the heart of the resistance.

But Trump's provocations in his battle with California have continually turned up the dial on Democratic enthusiasm here -- creating the possibility of much higher turnout than a normal midterm year.

"Trump has tried to make a political case that might work in a whole bunch of other states by confronting California, but it doesn't work in California," Carrick said. "We know that Democrats are on popular side of those issues in California," he added, ticking off topics like offshore oil drilling, immigration and fuel economy standards.

A poll last year by the Public Policy Institute of California found that 69% of California adults opposed more oil drilling off the coast, while 25% supported it.

With a close eye on Trump's encroachment on those kinds of policies in California, many newly-minted activists held monthly rallies outside Issa's offices in Vista. At those events, activists told CNN that the possibility for seven congressional pickups offered them a way to channel their despair about Trump into door-knocking and organizing.

They were thrilled when Issa was among the first prominent Trump allies to announce his retirement. His open seat is looking like a good prospect for Democratic candidate Sara Jacobs, who had more cash on hand than any of her Democratic opponents at the end of the last filing period.

The independent group aligned with the Democratic Congressional Campaign Committee has spent more than a million dollars opposing Republican Rocky Chavez in that race—in the hopes that Sara Jacobs or one of the other Democratic candidates will face Republican Diane Harkey, their preferred opponent, in the general election.

Because of the proximity to Hollywood and the Harvey Weinstein case, as well as the backlash against the male-dominated culture of Silicon Valley, the #MeToo movement has particular salience in this state.

"It's the first time in history that women have an advantage on ballots across the country. People are really excited about women candidates; they represent change in a way that traditional candidates can't," Burton said.

Jacobs, the independently wealthy granddaughter of Qualcomm founder Irwin Jacobs, has an all-female campaign team and has often spoken about taking on the patriarchy.

Katie Hill, a 30-year-old homelessness activist who is locked in a close race with Bryan Caforio in Knight's district, filmed a widely viewed ad that featured her rock climbing.

"This can be hard," Hill says in the ad while rock-climbing in a canyon in the Santa Clarita area. "Not as hard as running for Congress when corporations are backing the other guy."

The surge in Democratic activism created more of a curse than a blessing for Democrats in one of the most unpredictable races here for Royce's open seat.

The leading Republican candidate, former Assemblywoman Young Kim, is expected to coast easily into the November contest. The longtime aide to Royce, who was born in South Korea, has highlighted her family's immigrant story as a bridge to independents and Democrats. Nearly 32% of district population is Asian, and 34% is Hispanic, according to census data.

"When my family came to this country, we came legally," Kim said in her first television ad. "And not because we wanted handouts but because we wanted the opportunity America provided to succeed on our own."

Kim's electoral strength immediately turned the primary race into a contest for the number two spot. It evolved into a nasty brawl between Cisneros and Thorburn. Cisneros won $266 million in the lottery in 2010; Thorburn, a one-time teacher who made his fortune selling insurance, mainly to teachers working abroad.

The DCCC intervened on behalf of Cisneros, a former Navy officer who they selected for their "Red to Blue" list. To elevate his chances, the DCCC's independent arm has spent the final weeks battering the lower tier Republican candidates: Nelson and Huff.

But the sheer number of candidates has made it virtually impossible to predict a winner in the 39th Congressional District.

Kim is one of seven Republicans on the June 5 ballot. Six Democrats running for the seat, two American Independent Party candidates, and two candidates who declined to state a party preference.

A changing California

Republicans have held a lock for decades on congressional seats in Orange County, the once-conservative birthplace of the new American right. In 2016, Clinton became the first presidential nominee of her party to win the county since the 1930s.

An array of demographic shifts has made it harder for Republicans to hold on to seats like that of Rohrabacher, Royce and Issa, who represents portions of Orange and San Diego counties. Over the years in the Golden State, Republicans have shifted from being the party of the suburbs in California to the party of eastern and rural California.

Democrats have heavily targeted younger Latino and Asian families in Southern California, leaning hard into registration on college campuses in places like Irvine where they hope to knock off Congresswoman Mimi Walters.

The seven Republican districts have also been transformed by socio-economic changes.

As housing prices have gone up along the coast and in Los Angeles, many millennials and younger Democrats are moving into suburban areas in the districts of Royce, Issa and Congressman Steve Knight, who represents more affordable areas north of Los Angeles like Palmdale and the Antelope Valley in the 25th Congressional District.

How Democrats fare in the suburban areas of these bellwether districts on June 5 will also offer clues about whether Republicans can hang on in November.

The extent to which low-propensity voters turnout on Election Day will speak volumes about whether Trump's Washington soap opera, and his cutting rhetoric, has permanently alienated more moderate members of the Republican Party.

"I actually think California is a bellwether for the whole country," said Bill Burton, a former Obama aide and Democratic strategist who is running the campaigns of strong women contenders like candidate Jacobs in Issa's district and Hill in Knight's district.

"You have a mix of suburban, ex-urban, rural areas where you have competitive races. I don't think it's that dissimilar from what you're seeing in place like Pennsylvania, or Minnesota, or Colorado."