This paper provides a theoretical model for the coattail e¤ect, where a popular candidate for one branch of government attracts votes to candidates from the same political party for other branches of government. I assume a political agency framework with moral hazard in order to analyze the coattail e¤ect in simultaneous presidential and congressional elections. I show that coattail voting is the outcome of the optimal reelection scheme adopted by a representative voter to motivate

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... ¤orts in a retrospective voting environment. I assume that an o¢ ce-motivated politician (executive or member of congress) prefers her counterpart to be a¢ liated with the same political party. This correlation of incentives leads the voter to adopt a joint performance evaluation rule, which is conditioned on the politicians belonging to the same party or to di¤erent parties. Two-sided coattail e¤ects then arise. On the one hand, an executive's success props up, while failure drags down, her partisan ally in the congressional election, which implies a presidential coattail e¤ect. On the other hand, the executive's reelection itself is a¤ected by a congress member's performance, which results in a reverse coattail e¤ect. JEL classi...cation: D72.