This thesis examines third-party actor influence on the domestic level in the ongoing diplomatic negotiations between P5+1 and Iran, with the research question being; What demands, arguments and strategies does the Congress and AIPAC use to influence the negotiation process between the U.S. and Iran? Secondly, what are their goals for doing so? Furthermore, the combined theoretical framework has been applied onto the material using a critical method in order to answer the questions. Legislative bills from the Congress, regarding congressional insight to the agreement, have been approved. In this pressured negotiation process with high stakes it is astounding that these actions are allowed. Due to this behavior from Congress, amongst others, it becomes interesting to study the negotiation process and its salient third-party actor influence. The results suggests that legislative actions combined with various demands and arguments, focused on mistrust of Iran, history and the security of Israel, are their main strategies to gain influence. An additional new, third, level has also been discovered by the authors of this thesis in regards to these complicated negotiations.