AbstractGlobally, 81 countries are now part of a customs union (CU), following the rapid proliferation of this type of trade agreement in past decades. Much of this growth has been driven by countries “upgrading” their links from a free trade agreement (FTA) to CU. At the same time, the rapid formation of new FTAs among countries that had no prior agreement in place has largely overshadowed this growth, making CUs the silent success of regional integration. Using the canonical regionalism model, augmented to allow for political bias towards firm interests, we investigate the endogenous choice of trade agreement. We show it is generally politically viable to move from FTA to CU, because such a move is rent‐creating; but for countries without a trade agreement in place, it may be optimal to form an FTA as a stepping stone to reduce the risk of political derailment. Importantly, forming a CU is consistent with member social welfare maximization: as long as trade with the rest of the world does not cease entirely, a CU leads to higher social welfare than either FTA or no agreement. These gains come at the expense of third‐country welfare. If past trends continue, one can expect more FTAs to be upgraded to CU with associated adverse consequences for outsiders.