This dissertation examines the role of incumbent ideology on the entry decisions of congressional opponents, particularly high quality opponents. In order to better understand the interplay between incumbent ideology and opposition entry, this relationship is investigated in three distinct types of elections: House primary, House general and Senate general elections. The findings strongly point to an advantage for clear ideological positioning in primary elections and in the majority of general elections. These findings have strong relevance for students of polarization and congressional behavior. Instead of ideological extremity being a detrimental strategy for incumbents, it serves to deter the entry of strong opponents for the majority of representatives and senators.