Mike Duhaime, a Republican strategist in New Jersey who has worked on national campaigns, said his fear is that “we’re shrinking ideologically as a party — and that’s a problem.”

“Some of the diversity of the party is leaving. If we’re going to be competitive in these suburban areas, if we’re going to take back Congress, it is going to have to be in these suburban areas,” Mr. Duhaime said. “And our brand is going to need to be more than just Trump.”

Nowhere was the fracture with the Trump-led Republican Party more potent than in a swath of wealthy suburban enclaves in the northeastern part of New Jersey, where Mikie Sherrill, a former Navy helicopter pilot and federal prosecutor, defeated her Republican opponent by more than 12 percentage points. It was a stunning margin in a seat that had been held for 24 years by Representative Rodney Frelinghuysen, a genteel, moderate Republican deal maker who rose to become the powerful chairman of the Appropriations Committee. The district had not elected a Democrat since President Ronald Reagan’s administration.

Ms. Sherrill’s victory was one of seven for Democrats along the so-called Acela corridor between Washington and New York: one in Northern Virginia, three in the Philadelphia region, three in New Jersey and one in New York City. In New York, Max Rose, a 31-year-old military veteran, ousted Representative Dan Donovan in a district centered on Staten Island, long a Republican stronghold and where Mr. Trump had remained relatively popular.

John McLaughlin, who served as a pollster for Mr. Trump in 2016 and who was involved in House races in 2018, blamed Republican congressional leaders more than the president for the losses in the region, citing the tax bill in particular as “a policy mistake that hurt many Republicans in the Northeast and others in high-income suburbs.”

While the measure cut taxes across the board, it placed a cap on the popular state and local tax deduction that is heavily used in the wealthier parts of high-tax states like New York and New Jersey. “The tax cut was good policy to grow the economy and create jobs, but when Republican leadership refused to negotiate a higher cap or phase it out over time, it made many suburban Republicans vulnerable,” Mr. McLaughlin said.