On average, across districts, roughly 6 percent of Obama voters switched and just under 6 percent of McCain voters switched; because there were more Obama voters than McCain voters in 2008, this means — as you’d expect — that more voters swung to the Republicans than to the Democrats. An additional 1.5 percent switched to third-party candidates.

But on turnout, the numbers were not evenly balanced for Democrats and Republicans. Only 65 percent of Obama’s 2008 supporters stuck with the party in 2010 and voted for a Democrat in the House. The remaining 28 percent of Mr. Obama’s voters took the midterm election off. By comparison, only 17 percent of McCain’s voters from 2008 sat out the midterms.

Turnout in midterm elections is always down from presidential elections, and Democrats routinely fight to return more of their voters to the polls than the Republicans. More Democrats come from groups, such as young people and Latinos, that typically vote at lower rates in midterm elections than other groups. But this 11-point difference in holding on to 2008 voters is larger than normal. It probably stemmed from a gap in enthusiasm between the parties’ voters in 2010, as survey data indicated.

It may seem hard to believe that the shellacking was more about who turned up than about who changed their minds between 2008 and 2010, but it lines up with a lot of other evidence about voters’ behavior. Most identify with the same political party their entire adult lives, even if they do not formally register with it. They almost always vote for the presidential candidate from that party, and they rarely vote for one party for president and the other one for Congress. And most voters are also much less likely to vote in midterm elections than in presidential contests.

These stable patterns of American politics reveal a clear path for both parties in 2014: Get your 2012 voters to the polls. Of concern to Democrats right now is that Republicans once again have the upper hand on enthusiasm going into November.

The 2014 fight is not over swing voters. It’s for partisans.