For much of the 20th century, each major party was a coalition of people who disagreed with one another. There were conservative Democrats, especially in the South, and liberal Republicans, especially in the Northeast and Upper Midwest, and many votes in Congress drew support from both sides.
That mix came apart over several decades. After the Civil Rights Act of 1964 and the Voting Rights Act of 1965, party loyalties in the South shifted gradually, and voters across the country increasingly chose the party that matched their views on a range of issues. Political scientists call this “sorting”: the parties became more internally alike and more different from each other.
Congress shows it clearly. Pew Research Center, using the DW-NOMINATE scores that place every member by how they vote, counted more than 160 moderates in Congress in 1971–72 and about two dozen in 2022. Both parties moved away from the center; the measure shows Republicans further than Democrats.
Sorting is not the same as anger
Sorting describes who belongs to which party. How people feel about the other side is a separate trend, measured with surveys rather than votes. Both have grown, but they are different things.
Sorting has a practical effect: with fewer members in the overlap, there are fewer natural partners for deals across party lines, and party leaders hold more of the agenda.
When you read about a vote in Congress, look at how many members crossed party lines and who they were. That number says more about how a bill was made than the final tally.