Electoral Margins and Political Competition

In this paper, we argue that recent trends in party seat margins and election vote margins result from structural changes in the nature of US political competition. We assemble a comprehensive database of electoral results for the House, Senate and presidential contests, from the 19th century until today. Seat margins declined in the recent period, so the margins of control of the House, Senate, and Electoral College by either party have become smaller. However, this was not accompanied by a decline in the margins of victory at the constituency level. We propose a model of electoral competition with multiple districts that can rationalize these trends. We show theoretically that an increase in politicians' information about voter preferences, together with the growing nationalization of politics, can account for the decrease in seat margins and the concurrent stability in vote margins. As implied by the model, we document that campaign contributions received by House and Senate candidates are increasingly concentrated in a dwindling set of swing constituencies.