Representative Mike Doyle of Pennsylvania, a veteran Democrat from a neighboring district, said Mr. Lamb had benefited from “buyer’s remorse” among Trump supporters and had wisely tailored his message to the conservative-leaning area.
“This guy has made a lot of promises that aren’t being kept,” Mr. Doyle said of the president.
Mr. Doyle said he had conferred with Mr. Lamb about the question of supporting Ms. Pelosi and urged him to “vote his district.” He suggested other Democrats in conservative seats might follow Mr. Lamb’s approach.
“Some parts of the country, that may be a good strategy,” said Mr. Doyle, the state’s senior-most Democratic lawmaker.
Mr. Lamb’s Republican opponent, Rick Saccone, meanwhile, campaigned chiefly as a stand-in for Mr. Trump, endorsing the president’s agenda from top to bottom. He campaigned extensively with Mr. Trump and members of his administration and relied heavily on campaign spending from outside Republican groups that attempted to make Ms. Pelosi a central voting issue. Conservative outside groups also sought to promote the tax cuts recently enacted by the party, but found that message had little effect.
Yet at midday on Wednesday, Mr. Saccone trailed by just over 600 votes with only a handful of ballots yet to be counted. He has not conceded and could conceivably challenge the results through litigation. In a meeting with House Republicans on Wednesday, Representative Steve Stivers of Ohio, who leads the party’s campaign committee, described the race as “too close to call,” according to a person who heard his presentation.
But Mr. Ryan and Mr. Stivers also called the election a “wake-up call” for Republican lawmakers, telling them that they could not afford to fall behind on fund-raising, as Mr. Saccone did.
Mr. Lamb raised $3.9 million and spent $3 million, compared to Mr. Saccone’s $900,000 raised and $600,000 spent as of February 21. But Republican outside groups swamped the district. Between conservative “superPACs” and the National Republican Congressional Committee, Mr. Saccone had more than $14 million spent on his behalf.
Mr. Lamb got just over $2 million.
As Republican lawmakers spilled out of their morning conference meeting, few seemed willing to come to grips with how much Mr. Trump is energizing Democrats and turning off independent voters. Some of them even argued that Mr. Saccone only made the race close thanks to the president’s rally in the district on Saturday.
“The president came in and helped close this race and got it to where it is right now,” said Mr. Ryan.
Others in the conference, however, talked more openly about the political difficulties of breaking with Mr. Trump.
“There is no benefit from running away from the president,” said Representative Patrick McHenry of North Carolina, a member of his party’s leadership, noting that Republican candidates need core conservative voters to show up and this constituency still backs the president.
“It doesn’t get them the same thing as Lamb opposing Pelosi,” said Mr. McHenry.
But Mr. Lamb’s margin and the turnout levels in the district’s suburban precincts proved crucial, and a handful of Republican House veterans conceded this broader vulnerability.
“We know that’s probably where the president’s appeal is the weakest,” said Representative Tom Cole of Oklahoma, a longtime party strategist, adding: “It’s a pattern we’ve seen throughout.”
But he argued that Mr. Trump’s unpopularity in high-income areas would not as adversely impact incumbents who have their own identity and are steeled for difficult races.
Yet even as most of the Republicans pinned the blame on Mr. Saccone’s fund-raising weakness or held up Mr. Lamb’s willingness to oppose Ms. Pelosi, refusing to fault Mr. Trump, one retiring lawmaker was more blunt.
“Denial isn’t just a river in Egypt,” said Representative Charlie Dent of Pennsylvania, a frequent critic of the president. “I’ve been through wave elections before.”
Democrats were buoyant as Mr. Lamb declared victory, viewing his upset as both a harbinger of a November wave and perhaps a sign that the party had overcome some of the most stinging Republican attack lines of the Obama years. Polling in both parties found Ms. Pelosi widely disliked among voters in the district, but the Republican ads featuring her failed to disqualify Mr. Lamb.
Pittsburgh Mayor Bill Peduto, a Democrat, said Mr. Lamb’s campaign showed that the Republicans’ anti-Pelosi playbook had real limitations. The race, he said, should embolden Democrats to contest difficult districts in the Midwest with an economic message that appeals to elements of Mr. Trump’s base.
“Conor Lamb was talking about redevelopment and economic growth, and the Republicans were talking about Nancy Pelosi,” Mr. Peduto said. “It’s like they couldn’t help themselves.”
Mr. Peduto urged the Democratic Congressional Campaign Committee, the electioneering vehicle for House Democrats, to expand its target list in the Trump-aligned Midwest.
“Look through the Rust Belt, in areas that used to be blue,” Mr. Peduto said. “If you’re in a congressional district that’s 8, 10 or 12 points carried by Trump, I would hope that the D.C.C.C. is now putting that in the target.”
To the extent that Democrats attempt further incursions into Trump country, it may test their party’s willingness to tolerate Lamb-like deviation on matters like gun control, and perhaps more widespread rejection of Ms. Pelosi.
Democrats in Washington have already faced criticism from liberal activists for intervening in primary elections, in states like Texas and California, to promote candidates that they view as more electable. Putting forward a slate of moderates in Republican areas could prove more controversial than boosting just one in a special election.
Up to this point, however, Democratic leaders and their campaign tacticians have taken a just-win approach — a far more permissive attitude than their Republican counterparts when it comes to Mr. Trump.
Clarke Tucker, a Democratic state legislator in Arkansas who is challenging Representative French Hill, a Republican, said on Wednesday that he took Mr. Lamb’s victory as a validation of a throw-the-bums-out message he planned to deliver in his own race. Campaigning in a seat that comprises Little Rock and its suburbs, Mr. Tucker said he seldom spoke about Mr. Trump and announced up front that he would not back Ms. Pelosi.
Mr. Tucker, who was aggressively recruited by the D.C.C.C., said he had told Democrats in Washington that he was “very frustrated with the leadership of the House in both parties” and no one attempted to dissuade him from delivering that message.
“That district is a lot like the one I’m running in,” Mr. Tucker said of Mr. Lamb’s seat. “I think voters are interested in changing the leadership in Washington.”