Our paper addresses why a moderate presidential candidate would select an extreme running mate, as was the case in the 2012 US presidential election. To address this question, we designed a one-dimensional policy game in which a moderate challenger uses their vice-presidential candidate as a policy tool to alter the median voter participating in the election. Our main conclusion is that the median voter is altered through a mobilization effect, by increasing own party turnout through the convincing of the more extreme segment of a party to participate in the election, rather than altering the voter’s decision to vote for a particular candidate. This decision function only has a marginal effect on independent voters. Our conclusion is in line with recent empirical advances in the literature and our paper aims to more formally ground these advancements in theory. We developed a set of comparative statics to apply the theory of running mate selection more broadly, germane beyond the case of the 2012 presidential election used to frame this topic. Our model also determines the optimal difference between the platforms of a challenger and their vice-presidential candidate.