Abstract: Citizens in contemporary democratic societies disagree deeply about the nature of the good life, and they disagree just as profoundly about justice. In building a social contract theory for diverse citizens, then, we cannot rely as heavily on the theory of justice as John Rawls did. I contend that Rawlsian liberals should instead focus on developing an account of constitutional choice that does not depend on agreement about justice. I develop such an account by drawing on the contractarian approach to constitutional choice pioneered by public choice theorists, especially James Buchanan. With some modifications, public choice can help identify mutually justifiable constitutional rules based on the extent to which these constitutional rules produce appropriate laws under normal conditions. This new, synthetic approach to constitutional choice also helps to explain the moral significance of contractarian agreement for the public choice theorist.