Huntington Beach, California (CNN) After the bus carrying Democratic gubernatorial nominee Gavin Newsom had pulled away, and the sound system was disassembled and the folding chairs collected, dozens of people remained behind following a Democratic rally under brilliant sunshine at a school here Saturday afternoon.

Surrounded by yard signs and flyers, they created an excited buzz in a meeting room inside the school building. Some stood in a semicircle around a young field organizer providing instructions on how to conduct door-to-door canvasses for Harley Rouda, the Democrat challenging longtime Republican Rep. Dana Rohrabacher. Others signed sheets to volunteer for local candidates for the State Senate, State Assembly and county sheriff. The largest group sat at tables in the center of the room handwriting postcards to targeted voters urging them to turn out in November.

Debra Valle, who relocated to Orange County from Michigan, sat behind a small pile of completed postcards. "We have been working for the Democratic Party since I moved here in 1985, and I usually felt I was out there alone," she said looking around the bustling room, as her husband, Tom Beck, stood over her shoulder. "So this feels great."

Target: Orange County

After years of neglect, Democrats are mounting their most serious effort in memory to crack Orange County, a vast (3.2 million population) and sprawling suburban county southwest of Los Angeles. Once a symbol of unbending conservatism and white racial backlash, Orange County has grown more competitive in presidential elections over the past decade, but Democrats have made strikingly few inroads in down-ballot races, from the US House to the state Legislature, county board of supervisors and city councils.

This year, Democrats are betting that constant turmoil surrounding President Donald Trump will allow them to establish their first real beachhead here. At the top of their target list are four Republican-held US House districts in the county that voted for Hillary Clinton over Trump in 2016.

"We have a chance to step up and step in, and repudiate Donald Trump and Trumpism," Newsom declared to ringing applause at a crowded rally later Saturday afternoon for Katie Porter, the Democrat challenging Republican Rep. Mimi Walters in nearby Irvine. At another point Newsom insisted: "Orange is the new blue."

The evolving nature of the 2018 electoral battlefield is quickly raising the stakes for Democrats in proving him right: Indeed, Orange County may be rapidly transitioning from an afterthought to a necessity for Democrats.

The most recent round of national polling has presented Democrats with an almost unbroken procession of good news: Amid the legal setbacks for former Trump lawyer Michael Cohen and former Trump campaign chairman Paul Manafort and the stories of internal White House chaos, roughly half a dozen surveys have found Trump's approval rating falling to 40% or below and Democrats opening a generally double-digit lead when voters are asked which party they intend to support for Congress in November. The latest CNN/SSRS poll was typical: It showed Trump's approval skidding to 36 percent and Democrats opening a 10-percentage-point advantage on the so-called "generic ballot" test.

Trump's core stays loyal

But those overall results obscured a starkly bifurcated situation. In the national CNN poll, Democrats enjoyed big advantages among nonwhite voters and benefited from a pronounced recoil from Trump among college-educated whites. Though exit polls had showed Republicans carrying those upscale whites by double-digit margins in the 2010 and 2014 congressional elections, the CNN survey found they preferred Democrats for Congress by 22 percentage points.

But Trump's core constituency of white voters without college degrees still preferred Republicans over Democrats for Congress by a resounding 57% to 38%. That strong tilt has been reconfirmed by the array of district-level House polls recently released by both Monmouth College and The New York Times in partnership with Siena College . Over the past few weeks, those two polling efforts have surveyed about 20 congressional districts in all parts of the country. Neither has found any Democratic House candidate leading among white voters without college degrees; only about a third of the polls found Democrats capturing even 40% of the vote among those blue-collar whites.

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Both the national and district polls show some erosion for Trump, even in these blue-collar places, particularly among working-class white women. That's opened opportunities for Democrats in a few blue-collar and non-urban seats, including districts in Iowa, Illinois and upstate New York. (Democrats are also generally showing more strength with blue-collar whites in both governor and Senate races in the key Rust Belt battlegrounds of Wisconsin, Ohio, Michigan and Pennsylvania.) But Trump's residual strength with blue-collar and rural whites suggests that Democrats may still struggle to capture more than a handful of seats dominated by such voters.

That increases the pressure on them to maximize their gains in white-collar suburban seats that have traditionally leaned Republican, including those in Orange County. "If Democrats aren't capitalizing on these California seats, and specifically the Orange County seats, then we've got a real problem," says Andrew Baumann, the pollster for Porter. "If the wave is forming like we think it is going to form, we need to be winning these seats."

Orange County's evolution into a battleground reflects both its own changes and the shifts in the two parties' electoral coalitions. In Ronald Reagan's re-election in 1984, he carried fully three-fourths of the vote here; as recently as 2004, George W. Bush won exactly three-fifths.

4 key districts

Democrats have improved their position since former President Barack Obama came close in both his races and Clinton carried it in 2016, becoming the first Democrat to do so since Franklin Roosevelt in 1936. They advanced on the same two currents that have carried white-collar suburbs toward Democrats all across the country: growing diversity (whites now compose only about 40% of county residents, about half their level in 1980) and improving performance among college-educated white voters, who are often right of center on fiscal issues but generally lean left on social issues.

But despite Clinton's breakthrough, Democrats failed to dislodge any of the four House Republicans holding seats in the county: Walters in Irvine, Rohrabacher in a coastal district, Ed Royce in an inland seat centered on Fullerton and Darrell Issa in a seat that straddles the southern Orange County/northern San Diego County border. Among them only Issa even faced a close race; none of the other three Democratic nominees spent even $100,000.

Good news, bad news for Democrats

This time Democrats have found strong candidates against the two Republican incumbents seeking re-election: Porter, the liberal and fiery University of California at Irvine law professor who is challenging Walters, and Harley Rouda, the more subdued former Republican entrepreneur running against Rohrabacher. With Issa and Royce retiring, Democrats have nominated environmental attorney Mike Levin for the former seat and Gil Cisneros, a lottery winner and Navy veteran, for the latter.

Money is cascading onto these four Democratic challengers, including from gilded Los Angeles liberal fundraisers who usually focus only on national politics.

"I have never seen this level of activity from the donor community around House races," says Andy Spahn, a longtime leading liberal fundraiser in Los Angeles. "People are writing and raising for House candidates with presidential election enthusiasm."

As the turnout after the Newsom-headlined rally in Huntington Beach showed, volunteer activity is also way up. And the OC House candidates are receiving unprecedented attention from party leaders: One week before Newsom, who's cruising to election as governor, held rallies with Rouda and Porter here, Obama chose Orange County for his first campaign trail event of 2018

But even with all this energy, several yellow lights are still blinking at Democrats here. One was the result of the June primary. Compared with 2014, the last midterm election, Democrats roughly tripled their turnout in the Royce and Walters districts, and increased it by a factor of about 2.5 in the Rohrabacher and Issa districts. Those were stunning gains. But the Republican candidates still received more votes than the Democratic contenders in all of those districts except the Issa seat.

Analysts note that the Democratic vote share is almost always higher in November than the June primary, but those results were a reminder of how deep the GOP DNA runs in these districts. Darry Sragow, publisher of the nonpartisan California Target Book, points out that since the new district lines were established in 2012, Democrats haven't carried any of the four Orange County US House districts in any races other than Clinton's victory, including state legislative contests and the 2014 gubernatorial race, when Democratic Gov. Jerry Brown romped statewide.

Sragow says that means enhanced Democratic enthusiasm alone won't be enough to tip these seats; that will require persuading independents and some Republicans too.

"It doesn't matter how much Democrats increase turnout in a district if there are not enough of them to outnumber the Republicans. And, based on election results for 2012, 2014 and 2016, an increase in Dem turnout, if it happens, is unlikely to produce a Democratic victory without more (gains with other groups)," Sragow noted in an email.

Another challenge for Democrats is minorities. They represent a large share of the vote in each of the four districts, but mobilizing them -- especially in a midterm election -- remains a challenge for the party: Nonwhite (and especially Hispanic) faces were conspicuously rare at both rallies Newsom headlined on Saturday. The county's large Asian-American population has also voted more heavily Republican than in most places: For the open Royce seat, Republicans have nominated Young Kim, a Korean-American former State Assembly member who both sides consider a formidable candidate.

And Republicans still have some powerful arguments to wield at the college-educated whites who have been drifting away from them. Most powerful may be opposition to a state gas tax that the party is seeking to repeal through an initiative this fall; though some of the Democratic House candidates have renounced it, the GOP is confident the issue will help them paint the Democrats as tax-and-spend liberals.

"Orange County may be more purple, but when you look at the college-educated whites they still fricking hate taxes," says GOP consultant John Thomas, who works extensively there.

The tide against Trump

But all of these Republican defenses could get swept away if the tide of discontent with Trump rises too high, Thomas acknowledges. His own polling suggests a significant drop in support for the President in the county since June. As elsewhere, Thomas says, Trump is holding support among working-class whites, but he's "hemorrhaging with white-collar whites." Says David Jacobson, a Democratic consultant working with Rouda: "I think people are underestimating how toxic Donald Trump is."

Trump's erosion has encouraged Democrats to challenge him more directly. While many Democratic challengers in competitive seats have been downplaying the President, Porter declares flatly in her ads that she will "stand up to Trump" and relentlessly highlights Walters' record of voting with the President on almost every issue. Even Rouda, who stresses his determination to work across party lines, drew his loudest applause at Saturday's rally by insisting that the biggest reason to replace Rohrabacher is his recent declaration that Attorney General Jeff Sessions should resign because he was not sufficiently loyal to Trump.

"We talk a lot about single issues, whether it's gun violence, health care, immigration, the environment, middle-class jobs shrinking," Rouda said in an interview. "All are very important. But what we're seeing really happen the last 60 to 90 days is (more focus on) this greater overarching issue about the threats to our democracy."

Strategists in both parties rank the four races in roughly the same order. Both sides agree that Democrats are best positioned to win the Issa seat (with Levin), and Rouda is slightly favored to oust Rohrabacher. The open Royce seat looks like a pure toss-up between Cisneros and Young, while the low-key but staunchly conservative Walters will be toughest for Democrats to beat, though hardly out of reach for Porter, especially if Trump's local standing doesn't improve.

CNN rates all the seats as Toss Up, except the one being vacated by Issa, which is rated Lean Democratic.

"If Trump can grow by 3 to 5 points in his approval (rating) between now and Election Day those seats can hold," predicts Thomas, the GOP consultant. "If he doesn't, my only clients winning will be the nonpartisan races out there."