The midterm elections on Tuesday laid bare the growing chasm between urban and rural America, leaving Republicans deeply concerned about their declining fortunes in the metropolitan areas that extinguished their House majority and Democrats just as alarmed about their own struggles to win over voters in states that strengthened the G.O.P.’s grip on the Senate.

For both parties, the election represented an acceleration of dizzying realignment along cultural lines. Districts that once represented the beating heart of the Republican Party rejected President Trump’s avowed nationalism as a form of bigotry, while Democrats further retrenched from the agricultural and industrial communities where they once dominated.

Democrats took control of the House not merely by making gains in coastal states that supported Hillary Clinton, but also by penetrating deeply into suburban corners of traditionally conservative states in the South and across the Plains, like Georgia, Texas and Oklahoma. The House results made clear that the Trump-induced difficulties Republicans are suffering with once-reliable voters are hardly limited to blue states and could make it substantially harder for the president to remake his upscale-downscale coalition in 2020.

“The party should be concerned when you look at what was once one of its bases and see how increasingly vulnerable we are with them,” said Gov. Bill Haslam of Tennessee, an old-guard Republican who heads the Republican Governors Association, pointing to Democratic incursions into localities like Cobb County, Ga., that were once conservative bulwarks.