Regional integration blocs are subject to the admission of new members, which must be approved by domestic institutions. This article analyzes how the incorporation of Venezuela and Bolivia into Mercosur passed in the Paraguayan Congress. While the first case lasted from 2007 to 2013, demonstrating parliamentary opposition, the second episode took place between 2015 and 2016, suggesting convergence between the executive and legislative branches on the issue. Using process tracing, the unveiled mechanism shows how government and opposition forces act to alter the duration of the bill in Congress and that political parties have a pendular behavior according to political cleavages. Moreover, the findings of this study suggest the existence of a parliamentary veto power in foreign affairs and the importance of having homogeneous coalitions to achieve faster approvals.