In recent years, US immigration policy has been driven as much by state politics and government as by national policy. Many states have become active in attempting to influence immigration within their jurisdictions: some have adopted “restrictive” policies—such as Arizona's SB 1070—that are designed to dissuade unauthorized immigrants and their families from residing in their jurisdictions; others adopt more accommodating approaches that seek to integrate immigrants and facilitate their employment. What explains these divergent responses? My hypothesis is that state immigration policies are a function of the “newness” of immigrant groups combined with the openness of state legislative institutions. The large-scale arrival of new immigrants generates external costs that are conducive to locally-based, nativist movements; however, restrictive policies are most likely to occur where these movements encounter low institutional barriers to legislative action. By contrast, states with recently assimilated immigrant communities are more likely to seek accommodating policies, even as rates of immigration increase. I use a multilevel statistical analysis of immigration policies enacted in all 50-states between 2005 and 2012 to test this hypothesis. The results show that states with recently assimilated immigrant communities—measured here via naturalization rates and minority legislative representation—were more likely to pass accommodating policies. Meanwhile, restrictive state legislation was more likely to be passed where: (a) local and county governments had previously enacted anti-immigrant legislation or partnered with the federal government in immigration law enforcement (via partnerships with Immigration and Customs Enforcement), and (b) where citizens had more constitutional authority to directly write, or overturn, legislation. Thus, nativism may find a receptive audience among state legislators, but only where legislators are less shielded from popular pressure and where fewer of them represent recently assimilated citizens.