Justice Brett Kavanaugh’s confirmation hearings jolted Republican enthusiasm. The July 22 NPR/PBS News House/Marist poll, for example, found that 78% of Democrats and only 68% of Republicans considered this midterm “very important.” By October, the same poll showed the Democrats’ edge had mostly evaporated: 82% of Democrats and 80% of Republicans called the elections “very important.”

There was a similar change in the generic ballot. In the Sept. 12 Quinnipiac poll, voters preferred Democrats to Republicans for the House by...