For the fourth straight year, no Senate Democrat was more conservative than a Senate Republican—and no Senate Republican was more liberal than a Senate Democrat. In the House, only two Democrats were more conservative than a Republican—and only two Republicans were more liberal than a Democrat. The ideological overlap between the parties in the House was less than in any previous index.

The ideological sorting of the House and Senate by party, which has been going on for more than three decades, is virtually complete. Contrast the lack of ideological overlap with 1982, when 58 senators and 344 House members had voting records that put them between the most liberal Republican and the most conservative Democrat; or 1994, when 34 senators and 252 House members occupied the same territory.

“The last couple of Congresses have been among the most polarized in history. This is just a continuation of that. There’s nothing that will break this [trend],” said Gary Jacobson, a University of California (San Diego) political scientist, who specializes in congressional politics. “Voters have been voting along party lines at the highest rate in 50 years; they expressed that vote at the congressional and presidential levels. It’s hard for members to win in districts where their party is not favored.”