Barry Burden, David Canon, Ken Mayer and Donald Moynihan have written this article for AJPS.  Here is the abstract:

State governments have experimented with a variety of election laws to make voting more convenient and increase turnout. The impacts of these reforms vary in surprising ways, providing insight into the mechanisms by which states can encourage or reduce turnout. Our theory focuses on mobilization and distinguishes between the direct and indirect effects of election laws. We conduct both aggregate and individual-level statistical analyses of voter turnout in the 2004 and 2008 presidential elections. The results show that Election Day registration has a consistently positive effect on turnout, whereas the most popular reform—early voting—is actually associated with lower turnout when it is implemented by itself. We propose that early voting has created negative unanticipated consequences by reducing the civic significance of elections for individuals and altering the incentives for political campaigns to invest in mobilization.