I write from time to time about the myth of the independent voter, which goes something like this: there aren't any. Oh, lots of people say they're independent, but it turns out that most of them lean in one direction or another, and when Election Day rolls around the leaners vote just as reliably as stone partisans. True independents—the ones who switch between parties from election to election—make up only about 10 percent of the electorate.
Still, 10 percent is 10 percent. It's not quite nothing. But it turns out that it really is. Today, Lynne Vavreck breaks things down a bit further and explains just how these folks vote:
Only a small percentage of voters actually switched sides between 2008 and 2010. Moreover, there were almost as many John McCain voters who voted for a Democratic House candidate in 2010 as there were Obama voters who shifted the other way....On average, across districts, roughly 6 percent of Obama voters switched and just under 6 percent of McCain voters switched.
So, yes, there are some true switchers. But mostly they're going to cancel each other out. The net result from a huge push for swing voters is likely to be no more than 2 or 3 percentage points. In a few high-stakes states in a presidential election, that might make them worth going after. But in your average congressional election, it's a waste of time and money. So what does make the difference?
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.
....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.
The problem is that going after turnout is every bit as hard as picking up the crumbs of the swing voters. Traditional Democratic constituencies—minorities, low-income voters, and the young—simply don't turn out for midterm elections at high rates. They never have, despite Herculean party efforts and biannual promises that this time will be different. But it never is. They'll vote for president, but a big chunk of them just aren't interested in the broader party.
So what's the answer? Beats me.