US Parties Sorted From Overlapping to Polarized

Through most of the 20th century, US parties overlapped ideologically—many Southern Democrats were right-wing and many Northeastern Republicans were left-wing, belonging to a party for tribal or historical rather than ideological reasons—but over time, as visualized through roll-call votes, they separated completely so there is now no overlap, producing en-masse party-line voting and removing the cross-party common ground that once enabled cooperation.