But starting in the 1990s, and accelerating after 2000, the preferences of old and young, and white and nonwhite, have separated more sharply. The change has come in two stages, starting in 1994. In the backlash against Bill Clinton’s chaotic first two years, white voters backed GOP congressional candidates by a resounding 16-point margin. And in every congressional election since, Republicans have outpolled Democrats among white voters, six times by commanding double-digit margins.

The second important change followed a few years later. After 2000, the political preferences of young and old voters rapidly diverged as the first members of the racially diverse, socially liberal Millennial generation (generally defined as those born after 1980) entered the electorate, and Democratic-leaning Franklin Roosevelt seniors were replaced by the more Republican-leaning Silent Generation and early Baby Boomers. As a result, congressional Democrats have run at least nine points better among young voters than among seniors in each of the past five elections (and at least 16 points better in the past two).

In other words, the racial and generational difference in participation between presidential-year and midterm elections is long-standing; it’s the more recent divergence in preferences that has resulted in the GOP’s midterm advantage. Other factors, of course, also shape the results in these off-year contests. More often than not, the party that won the previous presidential election loses seats in the subsequent midterm. When the incumbent president is unpopular (as Obama is now), his party’s losses are typically greater. And Senate results are always heavily shaped by the map of states on a given year’s docket. But distinct from all these cyclical factors, the electorate’s composition now stands as a structural advantage for the GOP in off-year elections. And in a year like this, when the midterm electorate’s customary whiter and grayer complexion converges with low approval ratings for a Democratic president and a Senate battlefield centered on red states, Democrats understandably feel as if they are caught between colliding storm systems.