International treaty negotiations and domestic politics are interrelated. We show that negotiators can strategically select treaties to mobilize domestic interest groups and support particular candidates for office. Interest groups will support (oppose) political parties that will ratify treaties benefiting (harming) them. By designing treaties that mobilize these "swing" groups, negotiators can affect different parties’ chances of assuming power. Our model produces specific predictions for treaty design, contingent on the preferences of negotiators, parties, and interest groups. With conservative and moderate interest groups, the results may be counterintuitive—for instance, foreign governments pushing liberal treaties on liberal incumbents they dislike, intentionally mobilizing interest groups against the treaty and in favor of conservative challengers. Highly competitive systems, particularly those where moderate interest groups’ support is in play, give treaty negotiators leeway to shape political outcomes. Powerful interest groups may exert strong influence or no influence, depending on the cost of appeasing them and whether domestic and foreign negotiators are willing to gamble the incumbent’s political future for the achievement of their policy ideals.