… is from pages 10-11 of James Buchanan’s 2002 essay “Madison’s Angels,” which is chapter 1 of James Madison and the Future of Limited Government  (John Samples, ed., 2002) (link added):
[A]n increase in the politicization of our society almost necessarily reduces the proclivities of persons to behave like angels. In a paper written a quarter century ago entitled “Markets, States, and the Extent of Morals ,” I suggested that, if we politicize activities that extend beyond our moral capacities, we necessarily generate increased exploitation. To put this argument in Madisonian terms, if we put too much reliance on politics, we may stifle even those behavioral motivations that might qualify as near-angelic. This conclusion becomes especially relevant as and if we allow our political units to become too large, in both membership and territorial extension. A returned James Madison would surely stand aghast at the behemoth that is the United States federal government.