The question of whether parries converge or diverge over time has attracted a great deal of theoretical and empirical attention. In this article we make two contributions to this literature. First, rather than looking at general measures of ideology, we examine a specific policy area-environmental policy-to see whether the parries have diverged or converged. We utilize ratings produced by the League of Conservation Voters to obtain measures of congressional voting. Unlike other issue-specific studies of divergence, we adjust these scores, using a methodology recently developed by Groseclose, Levitt, and Snyder (1999), to make them comparable across time. Our results show that Republicans and Democrats in Congress have diverged over time on environmental issues. Second, once we determine that the parties have diverged, we analyze the underlying causes of this divergence. We provide three explanations for divergence between the two parties, based on the fact that parties are not monolithic but rather are made up of regional, factional, and individual components. If regions behave differently on an issue, then shifting representation of regions within parties will lead to shifts in overall party behavior. When internal fractions with stronger views than the general party are more supported by interest groups and less constrained by issue salience or economic conditions, then the parties are more likely to diverge. And when party members are replaced by individuals with different views on an issue, overall party behavior shifts accordingly.
