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Almost every argument about whether America is "more divided than ever" is really an argument about measurement. There are at least three different things the phrase can mean — how far apart elected officials vote, how much ordinary partisans dislike each other, and how sorted the country is geographically — and they do not move together. Two of them are at record levels. One of them is much more complicated than the headlines suggest.
The anchor for all of it is DW-NOMINATE, built by Keith Poole and Howard Rosenthal and maintained at Voteview by Jeff Lewis and colleagues. It reads every non-unanimous roll-call vote in the history of Congress and places each member on a single left–right scale by asking one question: whose votes does this member's record resemble? Party labels are never used as an input. That is what makes it possible to put the 119th Congress and the 46th on the same axis and compare them directly.
Dimension 1 captures the main economic left–right conflict and explains the large majority of roll-call voting in most eras. A second dimension picks up cross-cutting conflicts — for most of the 20th century, race and civil rights. That second dimension has largely collapsed since the 1990s: nearly everything now loads onto the single partisan axis, which is itself a form of polarization.