Over the weekend, I was reading the book "The Gamble" by The Monkey Cage's John Sides and Lynn Vavreck, which explores why Barack Obama won reelection in 2012 with an emphasis on the numbers.

One passage jumped out at me: Sides and Vavreck found that Obama was able to count on strong party loyalty in that election. Over the course of the past few decades, partisans have been increasingly unlikely to vote against their party.

What was interesting, though, is that, to a lesser extent, that holds for independent leaners -- that is, independents that lean toward one party or the other. Sides and Vavreck:

They look much more like true partisans in terms of their voting behavior than they do pure independents. For example, in 2008, “pure independents” who reported voting in the presidential election split 51%–41% for Obama, with the remainder voting for another candidate. The vast majority of Democrats (90%) voted for Obama, and so did 90% of independents who leaned Democratic. Similarly, the vast majority (92%) of Republicans voted for John McCain, as did 78% of independents who leaned Republican.

The implications of that finding are big, because -- despite the growth in the number of people who identify as independent -- the vast majority still lean toward one party or the other.

Pew Research has data on party identification that stretches back to 1939. The recent increase in independent identification is obvious.

If you look at the period since 1990, it's not quite as stark, but still notable.

But, again, not all of those "independent" voters are what Sides and Vavreck call "pure independents." Most align with either the Democrats or the Republicans. When you isolate those groups, you can see that the number of independent-independents has remained fairly steady.

Gallup has more granular data from weekly polling over the past decade or so. In this view, the number of "pure independents" -- the yellow section -- has gotten a bit larger, but not much.

The implication of this? The persuadable middle in 2012 was much smaller than one might assume. And in future elections -- say, next year's -- leaning independents are likely to vote like the partisans they lean toward.