Polarization in Congress can be conceptualized in two ways: as the extent to which the parties vote as cohesive, opposed blocs; and as the ideological distance between the two parties.

In terms of voting behavior, Congress has become increasingly polarized over the past several decades. In the 1950s-70s, Democrats and Republicans voted with their party about 60% of the time when their respective party leaders were divided. By the 1990s, members of each caucus toed the party line more than 80% of the time on divisive votes, and lately it’s risen to more than 90% of the time (Lee 2015; CQ Roll Call 2022 Vote Studies). More sophisticated measurements of the same concept yield a similar trend over time (Canen et al 2023; Handan-Nader 2024; Mehlhaff 2024).

It’s harder to measure the extent to which the two parties have drifted apart in terms of their underlying ideologies. Since party ideologies aren’t directly observable, they’re often inferred from voting behavior or campaign contribution sources. The best available evidence using these techniques suggests that the median ideology for the two major parties at both the federal and state level have steadily diverged over the same time period as party-line voting has increased (Canen et al 2023; Handan-Nader et al 2024).