Instead, Democrats appear highly competitive in many conservative districts. Already, there are polls showing Democrats ahead in Kentucky’s Sixth District, West Virginia’s Third, North Carolina’s Ninth, New York’s 22nd and Montana’s at-large district. Mr. Trump won each by at least 10 points.

One possibility is that Democrats are unexpectedly putting conservative districts into play because the overall national political environment is more favorable to Democrats than the generic ballot polls imply. Another possibility is that a district’s presidential vote choice will play a smaller than expected role in determining how a district will vote for the House.

Indeed, there aren’t many polls showing Democrats excelling in the well-educated districts where Mrs. Clinton won. Polls sponsored by Democratic groups have shown Republicans leading in Illinois’s Sixth, Pennsylvania’s First, Washington’s Eighth and California’s 39th. Even in the well-educated districts where Democrats lead in recent polls, like Virginia’s 10th or California’s 48th and 49th, the polls show Democrats merely running even with Mrs. Clinton.