One of my obsessions in American politics is the way the nature of partisanship is changing — the way it's getting harder, more extreme, and more personal.

The transformation is easy to miss because, superficially, we had Democrats and Republicans 60 years ago, and we have Democrats and Republicans today, and haven't they always argued?

But here's a sentence that shows just how much has changed over the past half-century: "Pure independents were more stable in their party support across 2000–04 than strong partisans were across 1972–76 and about as stable as strong partisans across 1956–60."

The observation comes from political scientist Corwin Smidt, and it's worth dwelling on for a moment. It means that voters who told pollsters they were "strong" partisans in the '50s were about as likely to vote for the other party as "independent" voters are today.

That sounds ridiculous. But as Smidt writes, it actually makes perfect sense. What's changed here isn't the voters; it's the parties.

Why today's independents are so partisan

A paradox of modern politics is that 1) more Americans than ever are identifying as independents even as 2) fewer Americans than ever switch the party they vote for between elections.

Today's self-identified independents, in other words, aren't very independent — they're actually predictably partisan, at least in the way they vote.

One interpretation of this is that so-called independents are lying — they're not really independent, they're actually Democrats or Republicans in disguise.

But Smidt argues that the fault is in our parties, not in our voters. The two parties have become so polarized, and the choice between them has become so clear, that pretty much every kind of voter reliably votes for one party or the other. That's true whether they call themselves independents or partisans, and it's true even when they don't pay attention to politics. This is probably the key chart:

The data comes from the American National Elections Survey — a massive survey conducted after every election. One of the questions asks whether voters felt like they really understood the differences the two parties had on the issues.

What Smidt found, looking at responses to that question over time, was that the voters were becoming much more aware of how the two parties differed. The change was so sharp, he writes, that "independent and inattentive voters exhibit an awareness of candidate differences across more political issues than strong partisan or politically attentive Americans prior to 1980."

To put that more simply, a voter who mostly ignores American politics today is about as informed on the differences between the two parties as a political junkie was in 1980. That's an incredible finding.

The voters are right: The two parties really are different

The key point here, though, is that it's not necessarily the voters who have changed — it's the parties themselves that are different.

There was a time, not so long ago, when many elected Republicans agreed more with the Democrats than with other Republicans, and vice versa — and leading political scientists thought it a great crisis for our democracy. In 1950, the American Political Science Association's Committee on Political Parties released a report calling on the two parties to sharpen their disagreements so that the American people had a clearer choice when casting their ballots.

The political scientists eventually got their wish. According to the polarization measures kept by political scientists Keith Poole and Howard Rosenthal, party polarization is higher in modern Congresses than at any time since the late 1800s:

Voters — even inattentive ones — are seeing differences between the parties more clearly because those differences are bigger and thus more easily seen.

An example: In 1965, a Democratic president created a massive, single-payer health care system for the nation's elderly. But as liberal as Medicare was in both conception and execution, it received 70 Republican votes in the House as well as 13 Republican votes in the Senate. (This was the era in which Barry Goldwater had run for president exhorting Republicans to offer "a choice, not an echo.")

Obamacare, by contrast, was based off many Republican ideas (like the now-controversial individual mandate), relied on private insurers for the bulk of its coverage expansion, and ended up sacrificing its public option. But it didn't receive a single Republican vote in either the House or the Senate.

It's easy to see how a voter in 1965 might think Republicans were open to something like Medicare — particularly if they lived in a liberal area represented by a liberal Republican who actually was open to something like Medicare. Today, however, no voter would be confused as to which party supports Obamacare. The choice between the two parties is much, much clearer.

And that more or less solves the mystery of the loyal independents. "Polarization has not strengthened their sense of partisan loyalty," writes Smidt, "but the clarity of polarization has effectively allowed independents and the politically inattentive to act as loyal partisans."

The problem with clear parties

Polarization has become a dirty word in American politics. But it's exactly what political scientists were begging for in 1950 — they thought Americans needed a clearer choice, and now Americans have one. So what's the problem?

Smidt offers an interesting answer to this question. The differences between the two parties are so stark that there are basically no true swing voters left. The average rate of party switching over the past four elections was 6.2 percent — about half the average rate from 1952 to 1980.

But those inattentive and independent voters who were willing to switch between less distinct parties were important; they gave parties a reason to stick near the center and ensured they were punished if they severely mismanaged the economy. As they decline, parties can focus more on turning out their bases, and they are more insulated from the effects of poor governance.

The underlying theory here is counterintuitive. We assume that American politics is stronger if all voters are well informed and able to make rational decisions about which party to support (though, remember, politics makes even smart, well-informed people stupid!). But some political scientists, including Smidt, emphasize that American politics needs many different kinds of voters to function — and that includes voters who aren't very well informed about American politics, aren't particularly committed to either party, and don't really know what they want.

Polarization has made the differences between the two parties so huge that swing voters are a dying breed, and the electorate is increasingly split between those who strongly prefer the Democratic Party and those who strongly prefer the Republican Party. On some level, that's the dream — the two parties are really different, and voters really should know which they prefer. But those inattentive voters we long looked down on played an important role in American politics, and we may miss them when they're gone.