(AP Photo/Orlin Wagner)

American voters are becoming more reliably partisan. This is visible in lots of different evidence. Now, new research has nicely captured a logical consequence: the decline of the swing voter.

The research — “Polarization and the Decline of the American Floating Voter” — is from Michigan State political scientist Corwin Smidt. “Floating voter” is analogous to what many people mean by “swing voter”: someone who votes consistently but swings back and forth between the two major parties. The graph below compares the fraction of floating voters in consecutive pairs of elections to those who voted in both elections but stuck with the same party (“stand-patters”), who voted only in one of each pair of elections (“surge and decliners”), and those who didn’t vote in either election (“repeat nonvoters”).

As you can see from the circled trend, the percentage of floating voters has declined from a high of 15 percent to 5 percent. Meanwhile, the consistent partisans, or “stand-patters,” have become more prevalent.

What is driving the decline in swing voters? Smidt shows that voters are increasingly cognizant of the sharp differences between the Republican and Democratic parties. In fact, a politically inattentive and unengaged American today is as likely to perceive important differences between the parties as a very engaged American was in 1960. See this graph:

In other words, the polarization of the two parties is so apparent that you can basically not follow politics and still perceive it.

Smidt goes on to show that people who think the parties differ tend to have opinions, and more consistent opinions, about the presidential candidates. Taken together, all three of these factors — perceiving the parties as different, having opinions about the candidates, and having consistent opinions — are related to the decline of the swing voter.

In short, there is less reason to “swing” when only one party is attractive to you.

Smidt sums it up like this:

By making it easy for Americans to recognize party differences, polarization has reduced ambivalence and indecisiveness and provided a strong and consistent ideological anchor to Americans’ presidential preferences across time, even for independents and the less aware.

One implication, Smidt notes, is that American voters should be less responsive to election-year forces, like shifts in the economy or other important events. Another is that politicians have less incentive to appeal to swing voters and more incentive to appeal to their loyal supporters.

Of course, shifts in the economy and swing voters could be decisive in 2016 if the election is close enough. But most American voters will supply little in the way of drama or excitement. Predictable partisanship is increasingly the norm.