I don’t think it contradicts the text of Matt’s post, but it’s worth noting that the trend towards unilateral executive power is a question of constitutional norms, as opposed to the text of the Constitution. John Yoo’s nonsense notwithstanding, the framers were worried about the President having excessive authority over foreign affairs, and clearly divided the relevant powers between Congress and the President in ways that put substantial limits on the president’s authority. The trend toward unilateral power has happened because Congress has been for the most part willing to delegate its powers to the executive branch.
Madison was right about one important institutional question: in and of themselves, parchment restrictions on state power are not very effective constraints, and hence they had to be accompanied by an institutional design that would make such limitations effective. Where Madison has largely turned out to be wrong is in his assumptions about the separation of powers. Madison assumed that institutional actors would be very jealous about guarding their prerogatives. But in practice, rather than maximizing their authority members of Congress often take advantage of the separation of powers to evade responsibility. Thus dynamic explains not only the increased foreign policy powers of the presidency, but the general growth in policymaking authority on the part of both the executive branch as a whole and the federal courts.