One week ago, three distinct but overlapping currents combined to shift tidal forces in America's midterm elections — and smash the Republican Party's congressional fortress.
Those currents were demographic change, Democratic mobilization and disaffection with President Donald Trump. As is clearer now than on election night, they produced a blue wave that has fundamentally altered the political calculus of the capital.
The demographic changes follow long-established trends. Year by year, the share of whites in the population shrinks while the share of Latinos, blacks and Asian-Americans swells.
Educational attainment keeps rising, especially among women. Declining marriage rates leave a larger proportion of Americans living as single adults. The rural population declines, while urban and suburban areas grow.
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.
The Democratic margin among Latinos grew from 26 percentage points to 40; among women, from 4 percentage points to 19, among single voters from 13 points to 24. The party's 3-point 2014 deficit among college graduates became a 20-point edge.
Voting for Democratic candidates ticked up a hair among blacks, Democrats and even Republicans. After losing independent voters by 12 points in 2014, Democrats won them by 12 points this year.
Having recaptured the Senate in 2014, Republicans hoped to pad their margin in 2018 by routing Democratic incumbents in conservative states Trump won easily. They did that in Indiana, Missouri and North Dakota — but Democrats flipped GOP seats in more evenly divided Arizona and Nevada.
Republicans entered the campaign as favorites to retain the House. The redistricting that followed the red wave of 2010 had fortified GOP incumbents.
Democrats began with just 25 prime targets — Republican-held House districts that Hillary Clinton had carried for president in 2016. Republicans had entered the 2010 contest with twice as many comparable Democratic targets.
The House Republican seawall was built to withstand a 23-seat battering. But the wave those three currents produced crashed so far over it as to position Democrats for a gain of up to 40 seats once vote-counting concludes.