Judge Glock, "The Mismeasurement of Polarization," National Affairs, Fall 2021:
It has become a commonplace of political discourse among academics, pundits, and politicians to claim that one or the other party has become more extremist, or that it has taken a radical position on some particular issue. Most often, those making the charge fail to distinguish the baseline against which the other side has supposedly radicalized.
When those on the left say the right has radicalized, they tend to measure it against the perceived will of the people at the current time, or against some new status quo — such as the Affordable Care Act — that was itself considered leftward radicalism just a few years earlier. When those on the right say the left has radicalized, they tend to measure it against some point in the distant past, or against the long-standing status quo.
What both measures of party polarization fail to take into account are long-term shifts in policy and public opinion that have occurred over America's history. When we look to these changes, it becomes clear that the baseline against which partisanship is measured has not remained static; instead, it has shifted to the left. Both parties have followed this shift over time, with Democrats largely leading the way and Republicans following from the rear.
This changing baseline helps explain why both parties can plausibly claim that the other side has radicalized. For the left — whose adherents measure polarization from the baseline of current policy and opinion, which has itself moved left — Republicans may appear extremist, even if their positions have remained fixed over time. For the right — whose adherents measure polarization against policies and opinions of the distant past, which fell more to the right than they do today — Democrats have radicalized, in the sense that they have followed the shift of policy and opinion to the left of where it once was.
On the whole, American politics has moved, and continues to move, along with policies and the preferences of the American electorate. This may be cold comfort for conservatives, who have witnessed both drift decidedly to the left over the past century and a half. But it can help assuage the alarmism over one-sided polarization that has long plagued our political discourse.
The Poole-Rosenthal scores provide a distorted view of ideological change over time.
Frances E. Lee, " Patronage, Logrolls, and “Polarization”: Congressional Parties of the Gilded Age, 1876–1896," Studies in American Political Development 30, Issue 2 (2016). The abstract:
According to the quantitative indicators scholars use to measure political polarization, the Gilded Age stands out for some of the most party-polarized Congresses of all time. By contrast, historians of the era depict the two major parties as presenting few programmatic alternatives to one another. I argue that a large share of the party-line votes in the Congress of this period are poorly suited to the standard conceptualization as “polarization,” meaning wide divergence on an ideological continuum structuring alternative views on national policy. Specifically, the era's continuous battles over the distribution of particularized benefits, patronage, and control of political office make little sense conceived as stemming from individual members' preferences on an underlying ideological dimension. They are better understood as fights between two long coalitions competing for power and distributive gains. In short, the Gilded Age illustrates that political parties are fully capable of waging ferocious warfare over spoils and office, even despite a relative lack of sharp party differences over national policy.