In the US political process, interest groups play an important role in shaping legislation. They provide votes and campaign contributions for political candidates but also other important organizational support, supply information insight, and mobilize voluntary campaign staff. The increase in official lobbying spending from $1.45 billion in 1998 to $3.37 billion in 2017 shows the importance of lobbying in the United States. To analyze interest group involvement is, therefore, essential to a better understand- ing of the political processes in a democratic system. In respect to US foreign policy, many studies address the impact of interest groups and their potential to affect policy outcomes. However, the nature and the role of interest groups in shaping US foreign policy remain ambiguous. This study focuses on the potential influence of interest groups on policy decisions in US-Iranian relations. The aim of this work is to explain what changed in regard to the role of interest groups in the most significant shift in US-Iranian relations under President Barack Obama before and after the vote on the Joint Comprehensive Plan of Action (JCPOA), which was intended to halt Iran's non-peaceful nuclear program in return for an easing of sanctions by the Congress. On 14 July 2015, the JCPOA was reached between Iran and the P5+1 (the permanent members of the United Nations Security Council plus Germany) and incorporated into law by the United Nations Security Council Resolution 2231. In order to avoid congressional approval, President Obama had defined the agreement as a political commitment among the nations involved instead of a treaty. However, due to congressional pressure Obama had signed a compromise bill in April, the Iran Nuclear Agreement Review Act of 2015, which enabled the legislative to review any deal with Iran before the lifting of US sanctions. To override a potential presidential veto in case of a congressional disapproval of the deal, a two-thirds majority would have been easily accomplished in the traditionally bipartisan institution in favor of a confrontational strategy towards Iran. But the debate on the JCPOA ended after a 30-day review period on 10 September 2015 with a failed vote on the resolution of disapproval of the deal in the Senate due to a Democratic filibuster; in the House of Representatives, the majority approved the same resolution, i.e. the disapproval of the deal, with an unprecedented 94% of the Democratic members in favor of the new strategy. To explain the reasons for the sudden divide by party affiliation on Iran is particularly interesting because it occurred without a change in conduct by the Iranian regime or a strong development in US-Iranian relations. In the past, the overwhelming strength of the interest groups supporting a hawkish stance on Iran sanctions in the US Congress had been helpful in exerting pressure on legislators but was not sufficient to prevent the JCPOA. To understand what conditions changed this study presents a systematic examination of a variety of factors linked to lobbying success in US foreign policy with Iran during the times of rising US sanctions in the 110th Congress period (2007-2008) to the path towards diplomacy in the 114th Congress period (2015-2016). It is one of the very few factor- centric approaches that not only includes a variety of factors that can be linked to lobbying success but also different interest group types like ethnic lobbies, peace groups and, partly, business lobbies. This study makes two important contributions. First, since it is not a single factor alone that is responsible for policy outcomes, this study contributes to our knowledge about how structural variables, issue salience, and actor strategies work together to produce outcomes. Second, this study provides a better understanding of interest group influence by identifying conditions under which interest groups can be successful in changing the policy status quo. The tested factors also promote the development of a standardized method to investigate lobbying success. The core argument of this study is that groups support legislators with limited resources by providing expertise and assistance to pursue a common goal. In the case of US-Iranian relations, the findings of this study show that the interest groups favoring a diplomatic solution were successful for a series of reasons: their alliance with President Obama in combination with increased financial contributions and organizational strength due to the combined resources in a strong coalition, as well as assistance by the media to spread their message. All these factors in combination with favorable policy windows created by the international setting balanced out the past disadvantage of groups favoring a diplomatic solution in respect to those favoring sanctions legislation and provided opportunities to break from the status quo.