Why is it that some countries have witnessed significant increases in inequality since the 1960s while at the same time experiencing very little change in the way politics is conducted? And why is it that in other countries, where inequality has increased much less, the Left has become substantially more redistributive? The answer, the authors argue, has to do with the interaction between inequality and political mobilization of low-income voters. The authors make two points in this article. First, high levels of inequality move Left parties to the left. Second, although increasing inequality pushes the core constituencies of Left parties to the left, it also makes some individuals less likely to be involved in politics. The authors argue that Left parties will respond to an increase in inequality only when low-income voters are politically mobilized. They explore these claims through a comparative analysis of Left party programs in 10 Organisation for Economic Cooperation and Development countries over the period 1966 to 2002.