We know that the ideologies articulated by the two major parties in America frequently evolve, but what causes these changes? In seeking to explain American party ideology development, many political scientists have focused on historical, economic, sociological, and psychological factors, but, ironically, very few have examined political sources of change. This chapter seeks to improve our understanding of American party ideologies by positing a new theory that focuses on a polity-centered factor: party control of government institutions. This theory is based on the premise that all government officials face incentives to exercise and expand the powers at their disposal. I theorize that when they do, their co-partisans face incentives to change their party’s ideology in a way that justifies the expansion and exercise of these powers. Conversely, those in the opposing party face incentives to change their party’s ideology in a way that criticizes the expansion and exercise of these powers. These changes in party theories of governance and party theories of intervention sometimes lead to enduring and significant transformations of American party ideologies.