IRVINE, Calif. (AP) — Once considered conservative holy ground, Orange County is starting to look like a last stand for California Republicans.

Chased out of much of California by Democrats who hold every statewide office and a 39-14 advantage in U.S. House seats, the party is trying to hold its ground in a place whose nickname, the Orange Curtain, recalls its famous Republican bona fides and where white, suburban homeowners once delivered winning margins for its candidates year after year.

But that's changed. And Democrats emboldened by an unpopular president and a diversifying population that favors their party hope to capture all four of the Republican-held U.S. House seats in the county.

Republican leaders, hoping to retain control of the House, have opened a 10,000-square-foot (929-square-meter) war room in an office tower near John Wayne Airport, filled with computers, phones and, on one day last month, dozens of volunteers making calls to potential voters in the June 5 primary election.

To Rep. Steve Stivers, who heads the party's campaign arm in the House, the Republican majority runs through Orange County.

It's in suburbs that "we are going to either hold the majority in '18, or lose the majority," the Ohio Republican said.

The risks are plain for Republicans in the state that is home to House Minority Leader Nancy Pelosi: Democrats dominate California politics, and midterm elections generally favor the party not in control of the White House. But President Donald Trump lost the state by more than 4 million votes in 2016 and there's no sign he's gained support since then.

"I'm profoundly concerned about my kids growing up in Donald Trump's America," David Min, the endorsed Democratic candidate in the battleground 45th District held by Republican Rep. Mimi Walters, says in an online video.

Nationally, Democrats have hopes of gaining 23 seats they need to take control of the chamber. They've been heartened by recent Democratic victories in Pennsylvania and Alabama, and House Speaker Paul Ryan's decision to retire at the end of the term has fanned the headwinds faced by the GOP.

The toughest fights in California are taking shape in seven Republican-held House districts carried by Hillary Clinton in the 2016 presidential contest, most of them in Southern California.

California Republicans are hoping immigration and a proposed gas-tax repeal will energize their voters.

Party leaders have been pumping money into the effort to get the tax repeal on the November ballot. Meanwhile, Republican elected officials in a string of cities and two counties — Orange and neighboring San Diego — have passed ordinances or taken other actions in opposition to the state's so-called sanctuary law, enacted by the Democratic-run Legislature in response to Trump's calls for more deportations and a wall on the U.S.-Mexico border.

For the GOP, there is a special urgency with the races in Orange County, an expanse of freeways, suburban sprawl and beaches long associated with conservative politics.

Richard Nixon's seaside estate known as the Western White House was in San Clemente, and county voters had supported an unbroken string of Republican presidential candidates reaching back to the Depression era, until Clinton broke through in 2016.

Losing one or more of the House districts would be a humiliating blow to a party that has been drifting toward irrelevancy in California for years. Independent voters are on track to soon surpass Republican registration, and Democrats boast a 3.6-million edge in voter registrations.

"Orange County is undergoing the biggest political challenge we've ever had," Republican National Committeeman Shawn Steel, who lives in the county, told volunteers at the GOP headquarters.

The plight of Republicans can be witnessed in voter registration numbers.

Ten years ago, the GOP held a 13-point edge in Orange County, but that's shriveled to 3 points while the number of independents, who tend to vote like Democrats in California, has climbed to 25 percent.

Why the shift? A surge in immigrants transformed California and its voting patterns. The number of Hispanics, blacks and Asians combined has outnumbered whites in the state since 1998. Meanwhile, new voters, largely Latinos and Asians, lean Democratic.

Those changing demographics transformed the 49th District, which includes parts of Orange and San Diego counties. Nine-term Rep. Darrell Issa is exiting after surviving by 1,600 votes in 2016. More than a dozen candidates are on the ballot to replace him, including Marine-turned-lawyer Doug Applegate, the Democrat who nearly won in 2016.

Arguably the toughest challenge for Republicans will be holding the 39th District, anchored in northern Orange County. Rep. Ed Royce, chairman of the House Foreign Affairs Committee, is stepping aside after 13 terms.

The district is about equally split in registration.

Republicans have established names on the ballot, including former state Senate leader Bob Huff, Orange County Supervisor Shawn Nelson and former state Assembly member Young Kim, a former Royce aide who has his endorsement.

The Democrats include several candidates that have invested heavily in their campaigns.

Gil Cisneros, a $266 million lottery winner and Navy veteran, has the look of the establishment pick and has loaned his campaign $2 million. He's been named to a select group of candidates by the Democratic Congressional Campaign Committee, the House campaign arm.

Former insurance executive Andy Thorburn, meanwhile, appears a favorite of liberals, with endorsements from nurses who want universal health care and Our Revolution, a Bernie Sanders-inspired political committee. He's loaned his campaign over $2 million. Pediatrician Mai Khanh Tran has pumped $480,000 into her campaign.

But 17 candidates will be on the ballot, creating tricky math for candidates, a challenge that extends to other districts with large fields.

In California, what matters is finishing first or second in the primary. That's because under election rules the top two vote-getters advance to November, regardless of party affiliation.

Meanwhile, some Republicans believe the threat is overrated for two incumbents in the county, Walters in the 45th District and Dana Rohrabacher in the 48th. Both won by double digits in 2016, even though Clinton carried their districts.

There are other risks. It's possible Republicans could be shut out of the November runoff in two marquee races — U.S. Senate and governor. That would almost certainly depress turnout, which would hurt Republican candidates down the ballot.

Last month, Republican volunteers hunched over their phones at tables covered, appropriately, in red, trying to tease swing voters into sharing telltale thoughts on the president and taxes.

On a message board, someone scribbled a portrait of the party's enduring mascot, an elephant, beside an urgent reminder: The election is one day closer.