Political scientists Boris Shor and Nolan McCarty have done more work on polarization in state legislatures than any other researchers in this decade. Their massive study of polarization can be seen in the Journal of Political Institutions and Political Economy, 2022, vol. 3, at pages 343-370. The title of their paper is “Two Decades of Polarization in American State Legislatures”. It finds the five most polarized state legislatures to be those of Colorado, California, Arizona, Texas, and Washington. See page 352. See the article here.

California and Washington state are the two states that use a top-two system. Proponents of top-two systems have always argued that top-two systems reduce polarization, but the date in the paper rebuts this idea.

Nebraska is the only state that has a non-partisan state legislature. Page 356 of the paper says, “Nebraska has polarized extremely rapidly.”