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.

区域一体化集团需要吸收新成员，并必须得到国内机构的批准。本文分析了巴拉圭国会如何通过委内瑞拉和玻利维亚并入南方共同市场。第一个案例从 2007 年持续到 2013 年，表明了议会的反对，而第二个案例则发生在 2015 年到 2016 年之间，表明行政部门和立法部门在这个问题上存在分歧。通过流程追踪，公布的机制显示了政府和反对派力量如何采取行动来改变该法案在国会的持续时间，以及政党根据政治分歧而采取摇摆行为。此外，这项研究的结果表明议会在外交事务中存在否决权，以及拥有同质联盟以实现更快批准的重要性。