The parties have become more polarized in recent decades, several academic studies have found. The demise of the conservative “Dixiecrats” in the 1960s and ’70s made the Democratic Party more liberal, and Republicans have moved even further to the right than Democrats have moved to the left, the studies show.

Elections like Tuesday’s suggest Democrats may be taking the Republicans’ cue, driven by the same activist forces that pushed them rightward. “In civics class in high school, you learn there are 435 members of Congress, and every one of them could lose in the next election. Now we’re down to less than 100 who can ever get beat in a general election,” lamented Representative Mike Ross of Arkansas, a Blue Dog co-chairman who is retiring from Congress this year. “So the Democrats run to their corner. The Republicans run to their corner, and as a result the country is being run by the extremes.”

“Redistricting,” he added, “has been bad for the country.”