LEESBURG, Va. — Emboldened by Donald J. Trump’s struggles in the presidential race, Democrats in Congress are laying the groundwork to expand the list of House Republicans they will target for defeat as part of an effort to slash the Republicans’ 30-seat majority and even reclaim control if Mr. Trump falls further.

Mr. Trump’s unpopularity, which has already undermined the party’s grip on the Senate, now threatens to imperil Republican lawmakers even in traditionally conservative districts, according to strategists and officials in both parties involved in the fight for control of the House.

Democrats are particularly enticed by Mr. Trump’s dwindling support in affluent suburban areas — including those near Kansas City, Kan.; San Diego; Orlando, Fla.; and Minneapolis — where Republicans ordinarily win with ease. Mr. Trump is so disliked among college-educated voters, especially white women, that he is at risk of losing by double digits in several districts that the 2012 Republican nominee, Mitt Romney, carried comfortably.

“It’s a remarkable situation. We, the Republicans, ought to be in a much stronger position in many suburban areas,” said Representative Charlie Dent of Pennsylvania, whose district includes both suburbs and small cities. “Because of the nature of the nominee, it’s going to be a lot more competitive than it ought to be.”