ANAHEIM, Calif. — Eileen Aispur tried to contain a bless-your-heart-level giggle as she listened to David Min, a Democrat, explain why she should consider him for a House seat representing this hill-ringed, steamy district south of Los Angeles. “We’re all Republicans here,” she blurted.
Still, he persisted. “I’m a fiscally responsible Democrat,” said Mr. Min, a law professor and one of a flock of Democrats seeking to leverage President Trump’s vulnerabilities to unseat House Republicans across the country. “If you don’t approve of the Trump agenda, I’m providing an alternative.”
After several losses in special elections in heavily Republican districts, Democrats’ efforts to win back the House are focusing largely on affluent suburban districts — in Florida, Illinois, Kansas, Pennsylvania and Southern California — where Hillary Clinton prevailed in her failed bid for the White House.
They are also focusing on districts where they hope to win back voters they lost to Mr. Trump last year, a knotty task that entails a pro-worker populist pitch in some districts, a pro-business, fiscal discipline pitch in others, and a careful pro-Affordable Care Act position in all.
“Some people have said our pathway to the majority is to do well in working-class districts where Trump was able to win last year and demonstrate to those voters that they have been sold a bill of goods,” said Achim Bergmann, a Democratic campaign consultant working on several House races. “But we also need to get to voters in districts that have not been traditionally competitive but voted against Trump, and are primed to support someone who will be a check on Trump.”
For their part, Republicans are looking for opportunities in Rust Belt states where Mr. Trump prevailed but House Democrats held on.
Midterm congressional elections tend to pivot largely on swing districts where Republicans and Democrats have roughly equal chances of winning. But with so few of these left, both parties are now relying on their own interpretations of which seats they can force into play, with Republicans largely on defense, as the party in power tends to lose seats in midterm years even when the president is popular.
While Republicans cling to a 52-48 majority in the Senate, Democrats in that chamber face difficult re-election campaigns in many states where Mr. Trump won — and scarce opportunities to win Republican seats.
The House landscape is different: Republicans there have been largely averse to confronting Mr. Trump, fearing the alienation of the president’s stalwart supporters more than the loss of disillusioned Republicans.
Democrats are betting that Republicans’ near lock-step allegiance with Mr. Trump, matched with an anemic list of legislative accomplishments in this Congress and traditionally low voter turnout in a midterm year when Democrats are energized, could make it happen for them. Democrats are also counting on Mr. Trump’s sinking approval rating, among all but Republican voters, to continue to fall.
“It is urgent that Democrats win the House in 2018 to restore financial stability and a path to the future for hard-working families,” said Representative Nancy Pelosi, Democrat of California and the House minority leader. “A Democratic victory is critical for the sake of the good health of the American people, the strength of our democracy and the future of our planet. Nothing less is at stake than America as we know it.”
In some places, like many districts in California, the Republican voter advantage has shrunk in recent years; in the district that includes Anaheim, where Representative Mimi Walters, a Republican, is seeking re-election, that edge has dropped from 43 percent in 2014 to just shy of 40 percent now.
“If Republicans are telling you they are on offense this cycle, they are delusional,” said Meredith Kelly, a spokeswoman for the Democratic Congressional Campaign Committee. “They had to spend $25 million to hold on to Kansas, Georgia, Montana and South Carolina. If they have to spend even a fraction of that money to defend their incumbents, they won’t be able to go on offense.”
Yet, after a bruising loss in the suburbs of Atlanta, Democrats have had to examine their playbook.
The Georgia race to fill the seat of Tom Price, the health and human services secretary, saw Republicans successfully cover the Democratic candidate, Jon Ossoff, with a bucket of Pelosi paint.
“The memories of her speakership and disapproval of her is so potent, and not just for the base,” said Matt Gorman, a spokesman for the National Republican Congressional Committee. “She turns off independents as well. It was consistent in Georgia 6 and other suburban affluent districts.”
Taking that lesson, Democrats are honing their messages to make them specific to districts, rather than sticking to the national party’s talking points, and steering away as much as possible from the struggle between the progressive base and moderate Democrats.
“There are a lot of people in this district who don’t like Trump but don’t like the national Democratic Party either,” said Mr. Min, who is one of a handful of Democrats hoping to unseat Ms. Walters, who is in her second term here and is closely aligned with Mr. Trump on contentious issues like health care.
The alchemy of message and candidate is always the hardest to master. To that end, from southern Michigan to Staten Island to here in Orange County, Democrats are fielding candidates with military experience — helpful in Republican-leaning districts — and those with health care backgrounds, from doctors to a neuroscientist to a woman who is emphasizing her experience as a breast cancer survivor.
“I am telling people I am not a Hillary person,” said Mr. Min, who has his eyes fixed carefully on nonaffiliated and Republican voters — Asians and parents in particular — who voted for Mrs. Clinton last year.
Republicans — and many election experts — say that even though midterm elections have historically been tough on the party in power, last year clearly demonstrated that voters make a distinction between congressional Republicans and Mr. Trump, especially when it comes to some incumbents like Representative Ed Royce, who represents a district near here. Voters cooled themselves at the Independence Day parade Mr. Min attended with paper fans festooned with Mr. Royce’s name.
Unhappiness with Mr. Trump’s policies “does not mean that those mainstream Republicans are willing to throw out every elected official,” said Nathan Gonzales, the editor of Inside Elections. “It is unclear whether voters now consider Trump and congressional Republicans under the same banner and hold them responsible for him.”
At the same time, trying to recapture independents and Trump-voting Democrats alone will not do the trick. “The fight for the House includes different battles in lots of different types of districts,” Mr. Gonzales said. “Democrats know they can’t compete in just the Clinton-Republican districts and take back the majority. There’s just not enough of one type of seat.”