John Sides has a few posts recently (here and here) in which he cites research by Eric McGhee at the Public Policy Institute of California on the effect of primary systems on the partisanship of elected officials. (Disclosure: Eric and I are working on a conference paper together on this topic.) His findings suggest that the effect of moving from a closed to an open primary is surprisingly modest. He further notes that during California's brief experiment with the blanket primary, politicians were no less polarized and the budget was still passed late.
Here's another way to look at this issue. In the graph below, I've plotted Jerry Wright's data on legislative partisanship in each of the state legislatures during the 1999-2000 session along the vertical axis. The horizontal axis is the type of primary the state was using at the time, going from most to least restrictive.
So what will happen if California adopts Proposition 14, the top-two primary, this spring? Well, I suppose that depends on just how representative we think Louisiana is. Does the Cajun State have a relatively moderate legislature because of its primary system, because of its unusual political culture, or because of other odd institutional rules in place there? Given that it's hard to separate those out, and given that other evidence about primaries and partisanship suggests little relationship between the two, I doubt the initiative would have anything close to the impact its backers suggest.
I'm wondering if anyone has recent data on Washington's state legislature, the members of whom have been elected through a top-two primary since 2006. Since Washington has been among the more polarized legislatures in recent years, it will be interesting to see if the chamber has been de-polarizing.