While James Buchanan’s simple insight that politicians are just as self-interested as the rest of us may have shocked the economic discipline, it strikes the rest of humanity as simple common sense. John Locke, writing well before the rise of Samuelson and Nordhaus, shows such common sense towards the beginning of chapter 12 of his Second Treatise:
And because it may be too great a temptation to human frailty, apt to rasp at power, for the same persons, who have the power of making laws, to have also in their hands the power to execute them, whereby they may exempt themselves from obedience to the laws they make, and suit the law, both in its making, and execution, to their own private advantage, and thereby come to have a distinct interest from the rest of community, contrary to the end of society and government: therefore in well-ordered commonwealths, where the good of the whole is so considered, as it ought, the legislative power is put into the hands of divers persons, who duly assembled, have by themselves, or jointly with others, a power to make laws, which when they have done, being separated again, they are themselves sunject to the laws they have made; which is a new and near tie upon them, to take care, that they make them for the public good.
That incredibly long sentence says two things, and both of them are true: legislators act in their own interest, and we should design our political institutions with that in mind to minimize the harm they can do. Buchanan would agree on both fronts.