This paper presents a formal study of economic influence by special-interest groups under political integration and separation. We first show that countries where more groups are organized to lobby gain from political integration on economic grounds. The reason is that a more organized country, under a political union, can affect policies in the other country to its advantage, something that a less organized country can do to a lesser extent. We then study the interaction of political integration and endogenous lobbying structure. We show that political integration affects the formation of interest groups. Moreover, if a country is more organized before political integration, this will continue to be the case afterward.