“I have some thoughts on ‘enthusiasm’ and the election,” tweeted Amy Walter, the Cook Political Report’s ace analyst of House races. What I and, I suspect, others expected to follow was a discussion of how voters’ enthusiasm, positive or negative, tends to determine who wins elections.

Especially in off-year elections, when turnout is more variable. Democrats had greater enthusiasm in 2006, and won both houses of Congress; Republicans had even more enthusiasm in 2010, and gained the largest number of House seats either party had conquered since 1948.