All those demographic trends expand the pool of Democratic-leaning voters. But they've been offset in recent midterm elections by low rates of voting among specific Democratic constituencies, most notably young voters and Latinos.
That's where the second current — Democratic mobilization — made a difference. In small but consistent ways, the party's campaigns altered turnout patterns in their favor.
Exit polls provide one basic measure. In 2010 and 2014, about the same number of Democrats and Republicans showed up to vote. This year, Democrats outnumbered Republicans by 4 percentage points.
The Democratic data firm Catalist, in a preliminary analysis of precinct, county and congressional district election results, adds more detail. It found the share of white voters dropped by 3 percentage points, from 79 percent to 76 percent, while the share of blacks, Latinos and Asian-Americans rose by 1 point apiece.
Similarly, the share of voters ages 18-24 and 25-29 each edged up by 1 percentage point; the share ages 30-39 ticked up 2 percentage points. The proportion of single voters and voters with college degrees swelled by 3 percentage points apiece.
Non-college-educated white men, the most Republican demographic group, dropped to 22 percent of the electorate from 25 percent in 2014; college-educated women of all races edged by two points. Rural voters fell by three points to 26 percent overall.
Those shifts occurred as turnout surged across the board. An estimated 49 percent of eligible voters cast ballots, up from 37 percent four years ago. Turnout of voters under 30, Tufts University researchers estimate, climbed to 31 percent from 21 percent in 2014.
Moreover, Democrats benefited from better-targeted turnout boosts. "They were more energized than Republicans in competitive races, but less so in targeted ones where their voters weren't as needed," says GOP pollster Patrick Ruffini.
The Trump factor powered Democratic mobilization and built on it. Alarm over Trump's policies and behavior propelled superior Democratic fundraising, which paid for late ads and turnout efforts.
At the same time, reaction to Trump made the Democratic-leaning constituencies that party strategists turned out even more Democratic-leaning than before. Thus exit polls showed that voters under 30, who backed Democrats by 11 percentage points in 2014, backed them by 35 points this time.