As the Democratic Party shifts leftward without much resistance, Republicans are fighting a war for the soul of their party. House Speaker John Boehner faces constant revolt on his right flank from a growing number of Tea Party-affiliated members who believe compromise is a dirty word. Outside conservative groups, such as the Club for Growth and the Senate Conservatives Fund, are enforcing ideological purity among members as well as primary candidates. Six of the 12 Republican senators up for reelection in 2014 are facing primary challenges from their right, even though several rank among the most conservative, according to the vote ratings.

Welcome to today's Congress, which in 2013 was more polarized than any Congress since National Journal began calculating its ratings in 1982.

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."

Beyond the polarization, the vote ratings highlight other compelling findings. Among Republican presidential candidates, Texas's Ted Cruz proved he could vote more conservatively than Kentucky's Rand Paul, 2013's Tea Party favorite. Senator Elizabeth Warren of Massachusetts is not quite as progressive as she advertised, at least compared with her Democratic colleagues. While most Democratic senators have moved to the middle as they face competitive elections, Al Franken has remained a stalwart liberal. Meanwhile, John McCain and Orrin Hatch, who had moved to the right in preparation for primary races, once again occupy the moderate wing of the Republican Party. Contrary to conventional wisdom, most members whose districts became safer during the redistricting process didn't become any more ideological.