Throughout US history, the two major political parties have changed positions several times on questions of how powerful the national government should be and how much it should regulate and guide the American economy. Are these ideological changes simply the product of historical contingency, or are there structural factors at work that can help explain these developments? This chapter finds that change in party control of government institutions can help explain change in party ideologies with respect to economic policy. Parties in long-term control of unified government tend to change their ideology in ways that call for a stronger national government and more economic intervention, while parties in opposition tend to change their ideology in ways that call for less national government power and less economic intervention.