Republicans in rural parts of Kentucky, Georgia and Tennessee were newly excited about the election, according to Democratic polling in those races, increasingly favorable toward the president and cheered by the recent confirmation of Supreme Court Justice Brett M. Kavanaugh. In suburban, wealthier parts of the country such as Northern Virginia, the opposite was happening, as moderates recoiled.

“Same president, same message,” said Democratic pollster Fred Yang, who was working for candidates in rural and suburban areas. “He sort of polarized the choices.”

Strategists from both parties say the president, in effect, erected a wall that broke the blue wave, allowing Republicans to hold onto key House seats and defeat Democratic Senate incumbents in conservative Missouri, Indiana and North Dakota. The same strategy, however, empowered Democrats to win decisive victories in formerly Republican suburbs in New Jersey, New York, Pennsylvania, California and the otherwise reliably red state of Texas.