Yet the depth of the Democratic losses in November cast doubt on that rosy thinking. For one, Republicans captured so many seats—both on the federal and state levels—that they can write their current advantage into electoral districts that will last a decade. And more ominously, a higher percentage of Millennials—that most crucial element of the future Democratic base—voted GOP than in the past. That's what led John B. Judis, looking at the voting choices made by white and middle-class Americans, to repudiate his prophesy of an emerging Democratic majority, and to declare in National Journal the onset of a Republican advantage.

The Pew study should provide some relief to Democrats even as it raises difficult questions for the country as a whole. The survey of more than 25,000 adults throughout 2014—a much broader sample than traditional opinion polls—found that Democrats retain a wide advantage among younger Americans. Fifty-one percent of Millennials (defined as people ages 18-33) either identify as Democrats or lean that way, compared to 35 percent who identify as or lean Republican. The gap isn't as wide as it was during Obama's first victory in 2008, but it is virtually unchanged from his re-election year of 2012. Democrats also expanded their edge with Generation X: Forty-nine percent of adults between the ages of 34 and 49 backed them, compared to 38 percent who supported Republicans. Democrats retained a significant lead among blacks, Hispanics, and women, and they held a nine-point advantage in overall party ID. (The study found that a higher percentage of people now identify as independents, 39 percent, than ever before.)

Generation Gap in Partisan Affiliation

Much of the Democratic edge among Millennials correlates with ethnic diversity: Forty-four percent are non-white, by far the highest percentage of any age group. Yet while black, Hispanic, and Asian American Millennials prefer Democrats by a margin of at least 35 points across all age groups, white Millennials are considerably more likely to back Democrats than are older whites.

White Millennials Are Divided in Partisan Leanings

The 2014 elections were decided by those who actually showed up to vote. But the Pew study was a survey of adults, not just registered voters, or those who cast ballots. And so the most striking aspect of the study is just how Democratic the population remained in 2014, even during a year in which Republicans swept elections nationwide. It highlights a striking gap between those who voted and the general population; the electorate was significantly older, and more Republican. Millennials, for example, made up more than 20 percent of the Pew survey but a smaller chunk of the voting public, according to exit polls. And party identification among 2014 voters was almost evenly split, despite the Democrats' longstanding advantage in surveys.