All seven of the House Republicans from California who represent districts that Mr. Trump lost voted for the bill, a collective act of political audacity in a state simmering with anger toward the president. While Mrs. Clinton won Representative Carlos Curbelo’s Miami district by 16 percentage points, he also voted yes. And other Republican lawmakers who represent districts that decisively rejected Mr. Trump, like Mr. Roskam and Martha E. McSally of Arizona, supported the measure.
All told, 80 House Republicans from districts Mr. Trump carried by 55 percent or less voted for the health law’s repeal. “Any Republican member of Congress in a seat that the president won by less than 10 points who isn’t concerned needs to be concerned,” said Glen Bolger, a Republican pollster.
Republican leaders felt squeezed between the rising popularity of the Affordable Care Act and the demands of their conservative base. In the end, they wagered that it was riskier to not fulfill their pledge to repeal the law — including some widely unpopular provisions like the mandate to obtain coverage — than to upend the medical care of millions of their constituents.
“We have to keep our own base excited because off-year elections are about the base,” said Representative Steve Stivers of Ohio, who oversees the House campaign arm. “We need to show them accomplishments.”
Republicans have their own opportunities in a scrambled House map, and they are aggressively recruiting in districts held by Democrats that Mr. Trump won or narrowly lost, including those represented by Rick Nolan and Tim Walz in Minnesota, Matt Cartwright in Pennsylvania and Jacky Rosen in Nevada. And many of the Republicans facing arduous re-election campaigns are among the most battle-tested.
Still, the Republican Party faces daunting challenges in 2018. Midterm elections are traditionally difficult for a party with full control of the government, even more so when it is bearing the burden of an unpopular or polarizing president. And Mr. Trump is struggling with disapproval ratings unseen this early in the administration of any other modern American president.
When a party wields its power to enact a controversial agenda — as in the first two years of Bill Clinton’s and Barack Obama’s presidencies — the losses can be immense. As Mr. Obama found out when his party lost 63 seats in 2010, making major changes to the country’s health care system is particularly perilous.
There is also the matter of which party has more energy, and liberal fury toward Mr. Trump is bolstering Democrats. Three liberal websites raised over $2 million in the 24 hours after the health vote. Republicans have already been forced to spend nearly $20 million on special elections trying to save a handful of conservative-leaning House districts.
Democrats are seizing the moment to seek out promising challengers, from blood-red Kansas to the blue-tinged suburbs of New Jersey and Pennsylvania, and enticing them with the prospect of a political wave. They have conducted focus groups in suburban areas of the Midwest, and are studying key voting blocs in California and New York, two states that could in theory furnish half the seats they need to take the House.
“The energy in the district is much, much different,” said Mayor Stephanie Miner of Syracuse, who has been approached by state and national Democratic officials about challenging Representative John Katko.
Democrats were in an eight-year defensive crouch under Mr. Obama, and they will have to overcome their recent dysfunction and division to make the sort of gains they are hoping for. But they are viewing 2018 differently. For three House cycles, Democrats operated within the narrow confines of a Republican-friendly map, hunting for the bare number of seats required to win the majority.
Many pick-up opportunities were hardly noticed. Last year, the Dallas district of Representative Pete Sessions, a veteran Republican, went for Mrs. Clinton, an indication of shifting demographics and suburban political sentiments. But Democrats had failed to field a candidate.
This time around, Democrats say they have a much broader strategy, focusing on more than 90 districts where Mr. Trump earned less than 55 percent of the vote. Strategists for the Democratic Congressional Campaign Committee have conducted a deep study of the 2006 elections, when Democrats last rebounded from defeat in a presidential race to seize control of Congress.
Reflecting the emboldened mood, formidable candidates have already indicated they are likely to run — even in districts that Republican incumbents have had little trouble holding.
This past week, a pair of Democrats emerged to challenge Mr. Sessions and Representative John Culberson, who represents another suburban Texas district that Mr. Trump lost last year. Representative Duncan Hunter, a Republican under investigation for financial improprieties, has drawn a retired member of the Navy SEALs, Josh Butner, as an opponent in a conservative-leaning San Diego-area district.
In Minnesota, Democrats are close to landing a prized recruit to run against Representative Erik Paulsen in an affluent Minneapolis suburban district that Mrs. Clinton carried by nearly 10 percentage points. The potential candidate, Dean Phillips, a philanthropist and the former chairman of Talenti Galento, said in an interview that the health care vote had left him close to entering the race.
“I woke up this morning, literally after yesterday’s vote, and knew that it’s decision time,” Mr. Phillips said on Friday. “My heart and my head are leaning in the same direction.”
Democrats are wooing a senior New Jersey legislator, John McKeon, to challenge Representative Rodney Frelinghuysen, a long-tenured Republican in a purple seat. And in New York, Ms. Hochul, a former member of Congress, has been approached by supporters about challenging Mr. Collins, a prominent Trump ally from the Buffalo area, according to Democrats in the state and in Washington.
In Kansas, Paul Davis, a former state legislative leader and gubernatorial candidate, is expected to run for the House seat being vacated by retiring Representative Lynn Jenkins, a Republican.
One challenge facing House Republicans is the fact that they have not received all of the assistance they had hoped for from the White House. Mr. Stivers, the Ohio congressman, said that during the transition, he shared with senior Trump officials a list of House Democrats in swing districts, suggesting that Republicans could flip the seats if the lawmakers were appointed to administration posts or named to ambassadorships.
Mr. Trump tapped none of the names on the Stivers list.
Republicans are also worried about the possibility that veteran lawmakers will follow the example of Representative Ileana Ros-Lehtinen of Florida, who last month announced that she was retiring. She represents a district that Mrs. Clinton won by nearly 20 percentage points.
Mr. Stivers said he had a list of about 10 House Republicans who may be tempted to retire, and whose seats could be difficult to hold. Though he declined to reveal which lawmakers were on the list, he said none of them had told him that they were retiring. At least not yet.
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