This paper studies why illegal immigration is widespread. We develop a political agency model in which a politician decides on an immigration target and its enforcement, facing uncertainty on the supply of migrants. Illegal immigration can arise for two reasons: the policy maker may be unable to enforce the target because supply is higher than expected; alternatively, he may underinvest in enforcement because of electoral concerns, and this occurs only when the incumbent and the majority of voters have different preferences over immigration. Empirical evidence provides strong support for our predictions, highlighting how electoral concerns shape illegal immigration flows.