The prospect has caused a geyser of candidates: As of Tuesday, 58 Democrats and 35 Republicans were seeking House seats, according to the Department of State.
Behind the altered political terrain are two restive groups of voters. College-educated suburbanites outside Philadelphia are angry at Mr. Trump and in a mood to send him a message. And blue-collar voters in western Pennsylvania are suddenly up for grabs, after they turned out strongly in the special election this month to elect a moderate young Democrat, Conor Lamb, over a Republican who ran as a wingman to the president.
In the Philadelphia suburbs, three Republican incumbents who have seen the writing on the wall are retiring, and a fourth is stepping down after a sexual harassment scandal. The latest to retire is Representative Ryan Costello, who announced his decision over the weekend, citing the difficulty of being heard over the cacophony from the White House.
The shifts outside Philadelphia mirror changes in suburbs around the country, as Mr. Trump’s policies and behavior have pushed away college-educated voters. The Pew Research Center reported this week that more voters with a four-year college degree now identify as Democrats or lean Democratic — 54 percent — than at any time since 1994.
Philadelphia’s suburbs were once so reliably Republican that Ronald Reagan’s cabinet included three residents of Montgomery County alone. But the region’s allegiance began to shift as well-educated and culturally liberal voters came to predominate in the suburbs in the 1990s.
The tide was held back in the last decade by gerrymandered congressional districts drawn by Republicans to protect their candidates. But that map was thrown out by the state court in January, and Republicans lost appeals to have the United States Supreme Court intervene. The state’s new districts hew more compactly to county and town boundaries.
Two nonpartisan handicappers of House races, the Cook Political Report and Sabato’s Crystal Ball, now rate four Republican-held seats in the Philadelphia suburbs as either “solidly” Democratic, “likely” to fall into the party’s hands, or “leaning” that way. The chairman of the Pennsylvania Republican Party, Val DiGregorio, declined to comment.
“This is a big wave coming our way,” said Representative Charlie Dent, a Republican from the Lehigh Valley north of Philadelphia, who is not running for re-election. “I’d get off the beach.”
Mr. Dent, whose seat is a potential Democratic pickup, predicted that the midterm elections will be a referendum on Mr. Trump’s conduct. “What’s hampering us is this never-ending drama and chaos that seems to emanate from the White House,’’ Mr. Dent said.
A fifth seat in the Philadelphia suburbs, based in Bucks County, is held by Representative Brian Fitzpatrick, a Republican whose new district voted narrowly for Hillary Clinton in 2016. Mr. Fitzpatrick has a record of attracting crossover votes from Democrats and enjoys the advantages of incumbency but is in his first term. “If you’re going to get incumbents, you typically knock them off in their first term,” Mr. Gerow said.
At the other end of the state, Mr. Lamb, a former Marine and prosecutor, is running his second race of the year, this time in a new district against an incumbent, Representative Keith Rothfus, a Republican who was once paired by a political blog in a celebrity look-alike feature with the cartoon character Milhouse.
The new district, the 17th, is more evenly balanced in its partisan leanings than the former 18th District, where Mr. Lamb won his upset victory in part by reawakening dormant Democratic DNA in white working-class voters who had supported Mr. Trump.
Mr. Rothfus, by contrast, is “several clicks to the right” of his new district, according to one Republican official who spoke on condition of anonymity to candidly assess the race.
Another Republican who was shifted to a more competitive district is Representative Scott Perry, who represents Harrisburg and York in central Pennsylvania. Mr. Perry is the only member of the far-right Freedom Caucus in the state’s Congressional delegation. Mr. Trump won his old district by 21 percentage points; the new map places him in territory the president carried by only nine.
Five Democrats are seeking their party’s nomination to run against Mr. Perry. Republicans predominate in the new district, but one strategist called them “softer Republicans,” many of them employed by state government. Mr. Perry may be out of step with many of his new constituents.
G. Terry Madonna, a political scientist at Franklin & Marshall College in Lancaster, Pa., said Democrats have the popular wind at their backs. In a poll of Pennsylvania voters he released on Thursday, 53 percent who are “very interested” in the election said they would vote for a Democratic House candidate, against 30 percent who planned to vote Republican. And Mr. Trump’s job approval rating in Pennsylvania is just 30 percent in the new poll.
“We’re seven months out, but it looks like there’ll be a Democratic wave,” Mr. Madonna said. The question, he said, is whether it will be modest or boat-swamping.