The race between Republican Cory Gardner and Democratic incumbent Betsy Markey for Colorado's 4th Congressional District was a partisan fight for political momentum. In the 2010 campaign cycle, Republicans looked to retake the historically Republican 4th District as part of a national strategy to win back the U.S. House, while Democrats tried desperately to hold on to both. Cory Gardner was only one of fifty-four Republican challengers to defeat a Democratic incumbent in 2010, but the Gardner-Markey race is particularly interesting as a case study of voter motivation and the mediating forces, both regional and national, that influence electoral success. Political commentators and staffers from both campaigns describe Markey's defeat as inevitable, but the same sources explain the election results from three different theoretical perspectives: (1) Betsy Markey was a poor fit for the district and never represented constituent interests, (2) Cory Gardner was the perfect candidate, and (3) Markey’s defeat was a result of the national political mood and a referendum on Democrats in Washington. This thesis analyzes and evaluates each of these theories, and concludes that a combination of the arguments and their evidence provides the most complete answer. While no single theory is the definitive reason that voters in the 4th District elected Cory Gardner, each contributes to a comprehensive understanding of the inevitability of Rep. Betsy Markey's defeat in 2010.