This paper provides a unified theoretical and empirical analysis of three longstanding explanations for the consistent loss of support for the President's party in midterm Congressional elections: (1) an electoral penalty by voters in midterm elections against the President's party, (2) a surge and decline in voter turnout, and (3) a reversion to the mean in voter ideology. To quantify the contribution of each of these factors, we build an econometric model in which voters jointly choose whether or not to participate and which party to support in both House and Presidential elections. Estimated using ANES data from both Presidential and midterm years, the model can fully explain the observed midterm gaps, and counterfactual simulations demonstrate that each factor makes a sizeable contribution towards the midterm gap.