The issue of political integration between two countries (more generally two political constituencies) for economic reasons is studied within the context of a simple endogenous growth model with a productive public good financed by taxation. We consider two countries which initially differ in terms of average endowment, size and inequality. Because taxation affects the distribution of income both within and between
countries, we are able to show how integration impacts it over the entire time horizon. The decision to integrate or not is made by the two national median voters.
We establish the net gain for any individual in any country derived from integration and offer two alternative decompositions of this gain. It is then proven that even though integration generates aggregate gains for both countries through an endogenous growth mechanism related to size, it may be in the interest of either median voter not to vote for integration given the transformation in the inequality schedule it implies. Surprisingly, even the poorer median voter may vote against integration. Turning to the process of union building, we prove that, once it is decided, integration
is irreversible. Countries may initially decide against integration, yet be willing to reverse this decision in a subsequent period.