Voters in midterm elections are different from voters in presidential elections: not only do fewer people turn out to vote when there isn't a presidential race on the ballot, but the people who do turn out to vote every two years are older and whiter than the presidential electorate. Vox's Ezra Klein explained this phenomenon — and why it helps Republicans — in our midterm election preview video:

But there's another important difference between the electorate in midterms and the electorate in presidential years. The people who care enough to turn out for a midterm election are more likely to be committed to a particular political ideology, and to the party that shares it. They think it's really important for the country that their side win — and the other side lose. That means that there are fewer swing voters turning out than in presidential years.

A study from the Pew Research Center shows that this is as true as ever in 2014. People who are solidly liberal or conservative are more likely to vote than people who are moderate, or not as ideologically committed. And because of that, the midterm electorate in 2014 overrepresents the ends of the ideological spectrum, and underrepresents the middle:

Voters aren't interested in crossing party lines

This means a couple of things for the candidates in next month's elections. For one thing, as much as some candidates in tough races (particularly Democrats) have tried to run away from their parties, most of the people who show up to vote in November will be voting for the party, not the candidate. Only ten percent of likely 2014 voters plan to "split their ticket" between parties — the rest are planning to vote down the ticket for Democrats, or for Republicans.

The above chart also shows how the ideological skew and the demographic skew of the midterms affect each other. Even though, generally, partisans are more likely to vote than non-partisans, straight-ticket Democratic voters (likely to be young or nonwhite) are actually slightly underrepresented among likely 2014 voters compared to all voters. Meanwhile, older, whiter Republican partisans are a lot more numerous than partisan Democrats among the people likely to vote in November despite making up a smaller share of registered voters.

No true independents

And if candidates are hoping to escape the partisan determinism of straight-ticket voting by targeting independents, they're setting themselves up for disappointment. The Pew survey reinforces something political scientists have known for years: even people who call themselves "independents" are more likely than not to vote straight down the ticket for one major party or the other. Among all voters, about a third of so-called "independents" are straight-ticket Republican voters, and a quarter are straight-ticket Democrats:

And that's among all registered independents — not just the ones who care enough to turn out in 2014. Those independents, like all midterm voters, are more likely to be consistently liberal or conservative — meaning they're more likely to be straight-ticket partisan voters than genuinely independent ones.