AbstractThis article addresses the influence of diaspora lobbies on US foreign policy by analyzing the failure of the paradigmatic lobby—AIPAC—to block the 2015 Iran deal. The literature on the efficacy of diaspora lobbies focuses on structural material factors. In contrast, this study introduces an agency-orientated constructivist approach focused on ideational factors. While the material institutional setting contributed to AIPAC's defeat by establishing a high bar to overcome, this was not insurmountable. Consequently, such material factors must be combined with ideational factors to fully explain AIPAC's defeat. In this vein, the prevalence of negative affective partisanship generated a “cultural opportunity structure” for the President to wield party loyalty to obtain the support of Congressional Democrats. Yet, this too was not insurmountable for AIPAC, had opposition to the deal not become tainted by partisanship. However, the “Republican first” strategy pursued by the public face of the campaign, Israeli Prime Minister Benjamin Netanyahu, undermined AIPAC's “discursive authority.” This generated “identity dissonance” within the American Jewish community and for other Democratic supporters of Israel, by casting their identification with Israel against their identification with the Democratic Party. In contrast, President Obama successfully framed the issue to minimize identity dissonance.