Argumentation is at the core of political communication. We study what criteria people use when they evaluate argument quality. We ask how justification content and argument source – who presents the argument – influence perceived argument quality, as well as how these two interact. Regarding justification content, we rely on criteria derived from deliberative democracy, and separate appeals to common or private interests. Regarding argument source, we study partial, impartial and reluctant sources. The promoted policy is in accordance with the interests of partial sources, it conflicts with the interests of reluctant sources, whereas impartial sources’ interests are not affected. We observed that appeals to common interest yielded higher perceived argument and justification quality compared to appeals to private interests. Our central observation was that sources did not influence perceived argument quality, unless arguments appealed to private interests. In other words, the influence of the argument source was contingent on argument content.