FOUNTAIN VALLEY, Orange County — Harley Rouda knows it took a little luck, a wacky opponent and a wave election for the 57-year-old Democrat to be elected to Congress last year in one of Orange County’s most reliably Republican districts.

Democrats here aren’t waiting around for any of that to happen again. They are registering voters at a furious pace to try to keep their historic flip of four Orange County congressional seats from being a one-hit wonder, and to help Democrats nationally hang onto control of the House that they took back from Republicans in 2018.

The result: There are now nearly 12,000 more registered Democrats than Republicans in Orange County. It’s a slim advantage in a county of 3.1 million people, but it’s the first time Democrats have held a lead of any size in 41 years.

But Republicans still hold a four-percentage-point registration advantage in Rouda’s district in southern Orange County, which includes the cities of Costa Mesa and Newport Beach. And Rouda, a former Republican who became a Democrat in 2017, says he’s been careful not to go too far to the left in Washington.

Instead, he said, his mantra is to play “inside the 20-yard lines. That’s where most of our voters are.”

The riskiest vote of his first year in Congress was his support of an impeachment inquiry of President Trump. That position is a no-brainer for a Bay Area Democrat, but not so in Orange and San Diego counties, where only 43% of likely voters approve of how Democrats are handling impeachment, according to a November survey by the nonpartisan Public Policy Institute of California. In the Bay Area, it’s 61%.

Rouda said he doesn’t second-guess his decision “because I don’t look at it through a political lens. I look at through a lens of ‘is this right or wrong?’ If I should lose because I did the right thing, I will sleep well at night.”

Other factors in next year’s election are beyond his control. Rouda knows he won’t have the good fortune of running against 15-term Rep. Dana Rohrabacher, as he did last year.

Rohrabacher, referred to by detractors as “Putin’s favorite congressman,” told The Chronicle in 2017 that U.S. intelligence agencies’ conclusion that Russia had interfered in the previous year’s election was “total bulls—” He also maintained that climate change was a hoax and that white supremacists who rampaged in Charlottesville, Va., had been secretly organized by a supporter of Hillary Clinton and Bernie Sanders.

Rohrabacher commonly won the district by 20-point margins — until last year, when Rouda prevailed with 54% of the vote. Rouda was hardly an overwhelming favorite. The California Democratic Party endorsed another candidate in the primary, but Rouda squeaked into the runoff by 125 votes and went on to win.

But now the Rohrabacher “advantage” is gone.

“We don’t have such a villain — in many voters’ eyes — to run against,” Rouda said. “His outlandish positions were certainly something that allowed us to separate ourselves.”

Instead of flying around the world to meet Wikileaks founder Julian Assange, as Rohrabacher did, Rouda has focused on constituent services, the unflashy part of being a member of Congress. His office boasts that during Rouda’s first six months in office, he responded to 31,114 constituent questions, resolved 189 of their cases and helped recover $170,000 in Social Security, Medicare and other federal money owed to people who live in the district.

“One of the biggest complaints we heard about Rohrabacher was about him not being present, not being seen,” Rouda said. So he is measuring success “in part with how we’re doing in constituent services.”

Being present in the district will be key because his likely opponent in November 2020 is a local officeholder, Orange County Supervisor Michelle Steel. The Republican has raised $1.1 million, including a $300,000 loan from herself, which is nearly as much as the $1.8 million Rouda has rustled in campaign contributions. Steel declined to be interviewed for this story, but her campaign offered supporters to speak on her behalf.

She is tight with Trump, thanks in part to her husband, Republican National Committee member Shawn Steel, a former chairman of the California Republican Party and a Trump supporter. When Trump landed in Los Angeles for a fundraiser earlier this year, two of the small clutch of people greeting the president on the tarmac were the Steels. Orange County voters will become familiar with the photo of their runway greeting, because Rouda’s supporters will be blasting it online and on TV ads nonstop until election day.

Orange County is still home to a lot of Republicans, and nearly three-fourths of its elected officials at all levels are members of the GOP. But Rouda’s camp is betting that, like suburban voters elsewhere, those Republicans are not happy with Trump’s brand of Republicanism.

“I think Trump helps us,” said Ada Briceño, chair of the county’s Democratic Party.

Michelle Steel, however, is hoping to tap into something that Rohrabacher couldn’t — Orange County’s changing demographics. Steel was born in South Korea and would be the first Korean American woman elected to Congress, something she hopes will win votes in a county with a growing Asian American population. In 2000, Orange County was 51% white and 14% Asian American. Now it’s 41% white and 21% Asian American, according to the 2017 American Community Survey.

“She’s a very down-to-earth lady. She’s Asian. She’s a mom, and she doesn’t like to raise taxes, and I like that,” said Hang Harper, a 42-year-old certified public accountant who lives in Fountain Valley and has gotten to know Steel at local Chamber of Commerce and health fair events. Hang, a no-party-preference voter who immigrated from Vietnam in 1993, spoke at the behest of Steel’s campaign.

Buoyed by their successes, Democrats activists will continue to try to flip Orange County’s voters one at a time. On a recent Monday, Jason Berlin led a group of activists registering voters at Saddleback College, a Mission Viejo community college that is a cross section of the county’s young voters.

Berlin, a former Los Angeles TV comedy writer, began registering voters in Orange County over the past year to push back against Trump. His organization, Field Team 6, now has 14 chapters in a half-dozen battleground states and has registered more than 15,000 voters. “I had to do something,” he said.

One of Berlin’s colleagues, Rebecca Ninberg, approached 19-year-old Sam Lum as he sat in an Adirondack chair on a campus quad watching an episode of “Friends” on his phone. Lum eventually registered as no party preference.

“She was really passionate,” Lum offered as his reasons for registering. “And, well, I kind of felt bad for her having to come here and do this. It can’t be easy.”

Joe Garofoli is The San Francisco Chronicle’s senior political writer. Email: jgarofoli@sfchronicle.com Twitter: @joegarofoli