Take a look at Boris Shor’s chart, posted at Fivethirtyeight by Andrew Gelman, showing the distribution of Republicans and Democrats on the left-right scale in each state’s legislature.
At the bottom of the chart is California. In no state are Democrats more to the left, Republicans more to the right, or the gap between the two greater. That’s polarization.
I wonder how much is simple difference of the parties’ social bases in such a diverse state, and how much is driven by the minority-veto provisions of the state constitution.