Why do governments choose multilateralism? We examine a principal-agent model in which states trade some control over the policy for greater burden sharing. The theory generates observable hypotheses regarding the reasons for and the patterns of support and opposition to multilateralism. To focus our study, we analyze support for bilateral and multilateral foreign aid giving in the US. Using new survey data, we provide evidence about the correlates of public and elite support for multilateral engagement. We find weak support for multilateralism and deep partisan divisions. Reflecting elite discourse, public opinion divides over two competing rationales–-burden sharing and control–-when faced with the choice between multilateral and bilateral aid channels. As domestic groups' preferences over aid policy diverge from those of the multilateral institution, maintaining control over aid policy becomes more salient and support for multilateralism falls.