Why do state legislators vote the way they do? Which influence is predominant: ideology, party, or public opinion? The implementation votes surrounding the Affordable Care Act (ACA) provides a unique setting to examine this question, as they make all three considerations highly salient. State roll call votes on ACA implementation were sometimes polarized and sometimes unexpectedly bipartisan. What accounts for the heterogeneity in individual legislator behavior on bills implementing the ACA at the state level? Using new data on legislator ideology and votes from 2011-2015, I show evidence that legislator ideology was by far the most important predictor of voting on implementation votes, far more so than legislator party or public opinion. Moreover, I show the influence of ideology is heterogenous by issue area and bill.