Contesting an International Environmental Agreement

International environmental agreements (IEAs) often condition entry into force on ratification by a minimum number of countries, yet deep environmental commitments frequently face strong domestic political resistance. We study how IEA breadth, through minimum ratification thresholds (MRTs), and depth are jointly determined when domestic ratification incentives are endogenous. In our model, lobbying competition between pro- and anti-environmental interest groups shape domestic ratification outcomes, and lobbying incentives depend on expectations about ratification in other countries. MRTs affect domestic political incentives by altering the pivotality of a country’s ratification for entry into force and the extent to which global emissions externalities are internalized. As a result, deeper agreements optimally feature lower MRTs: governments relax breadth requirements to offset endogenous domestic political resistance to more ambitious environmental commitments. Our analysis provides a political-economy foundation for the breadth–depth trade-off and offers a novel perspective on free riding that operates through domestic political effort rather than participation or enforcement mechanisms.