I argue that interest groups gain influence in national security politics by proving a "moral subsidy" --- legitimating third-party endorsement of the president's national security agenda. I deduce this claim from three assumptions: 1) interest groups gain influence by providing legislators with information about the consequences of their choices, and by educating constituents about policy changes and the actions of their representatives, 2) as argued in the "two presidencies" thesis, the executive initiates national security policy, and 3) the public pays little attention to foreign affairs. In most issue areas, interest groups have influence over both policy-makers and the public, but in national security politics the informational advantages they have over thinly-staffed legislators vanish when faced with the massive national security apparatus in the executive. However, they retain, even expand, their influence over the public, whose weak prior knowledge and beliefs make them susceptible targets of propaganda. The White House uses this public support to exert leverage over Congress to pass the President's agenda. I describe the legal and political constraints that motivate the executive to recruit extra-governmental organizations (EGOs), elaborate upon the concept by offering four main types of moral subsidy, and explain why the public relations campaigns in which they collaborate take on a distinctly moral inflection. I test these claims using a multi-methods approach. I present quantitative evidence that stronger congressional opposition to the president yields greater access to executive branch Federal Advisory Committees in national security affairs. I conduct a series of survey experiments investigating whether the proposed mechanism works as politicians intend. I construct an analytic history of moral subsidy in practice from the interwar years to the collapse of the Soviet Union. I also offer an argument about how EGOs use the access they gain to consolidate their influence, and explain how this practice has led to the emergence of a coherent network of powerful national security EGOs that I call the "credibility cartel."