2. The modern Democratic coalition is a boom-and-bust coalition that depends heavily on minorities and young people who turn out much less regularly in midterm than presidential elections. Older voters, who are trending steadily toward the GOP, vote much more reliably. Beyond any short-term factors, this is creating a structural disadvantage for Democrats in off-year elections: an electorate that is consistently older and whiter than it is in presidential races. As I wrote recently in The Atlantic: “In the five presidential elections from 1992 through 2008, exit polls conducted for a consortium of media outlets found that voters under 30 cast, on average, 18 percent of the ballots; in the five midterms that immediately followed those elections, young people accounted for just 12 percent of the votes. Voters over 65, by contrast, increased their share of the vote from 15 percent to 19 percent. The decline among minorities hasn’t been as consistent or as severe, but their share of the vote dropped two percentage points from 2004 to 2006, and three from 2008 to 2010, which are big shifts as these things go.”

Ironically, because Democrats have succeeded in turning out more minorities in presidential years, census figures show that the falloff in participation among Hispanics and African-Americans from an on-year to an off-year election is about twice as large as it was three decades ago. The most recent experience offered Democrats a daunting precedent: From 2008 through 2010, turnout dropped about one-third for African-Americans, almost two-fifths for Hispanics, and fully 55 percent for 18-to-24-year-olds, compared with about one-fourth for whites and only one-eighth for seniors. Tuesday’s election will help tell us how much the Democrats’ unprecedented efforts to identify and mobilize their core supporters can offset these underlying midterm trends, particularly in the battleground states both sides are targeting.