I previously posted new visualizations of political polarization in Southern American state legislatures in preparation for a conference honoring the work of legendary political scientist Keith T. Poole. Here’s those same plots for Western state legislatures from 1996-2017 (marked in 4 year intervals plus 2017 preliminary data). As always, polarization is measured as the distance between the Republican and Democratic party medians, averaged across the two chambers. Lines are colored according to which party is polarizing faster.
States in the West are the most polarized, and the fastest polarizing, according to our data. Colorado, California, and Arizona dominate the race to the poles, though other states too like Utah are quickly charging up the rankings. Colorado only in the past few years has overtaken California for the title of the most polarized state legislature in the country. Hawaii is clearly an outlier, with a liberal Democratic party paired with a liberal Republican party (in the House).
In terms of asymmetric polarization, here too, with the exception of Colorado, Democrats and not Republicans have been polarizing in more states.
Here are the positions of Republican party medians in the West. They’ve polarized quickest in Colorado.
The Democratic slopegraph is more dramatic. Dramatic falls are visible in almost every state except Alaska and Idaho.