Chris Cillizza notes that Republicans are responsible for this year’s uptick:

Republicans in Congress continued to grow more ideological over the first eight months of 2013 while Democrats’ partisanship stayed relatively steady. That’s in keeping with a long-term polarization trend in the House that began in the early 1980s. Since that time, both parties have grown increasingly more polarized in their votes, but Republicans’ polarization numbers have moved up further — and faster — than Democrats.