We offer a theory of motivated political reasoning based on the claim that the feelings aroused in the initial stages of processing sociopolitical information inevitably color all phases of the evaluation process. When a citizen is called on to express a judgment, the considerations that enter into conscious rumination will be biased by the valence of initial affect. This article reports the results of two experiments that test our affective contagion hypothesis—unnoticed affective cues influence the retrieval and construction of conscious considerations in the direction of affective congruence. We then test whether these affectively congruent considerations influence subsequently reported policy evaluations, which we call affective mediation. In short, the considerations that come consciously to mind to inform and to support the attitude construction process are biased systematically by the feelings that are aroused in the earliest stages of processing. This underlying affective bias in processing drives motivated reasoning and rationalization in political thinking.