[T]here is a key advantage that is often overlooked. Because [American] presidents are executives, and because of the discretion, opportunities, and resources available to them, they are ideally suited to be first movers and to reap the agenda powers that go along with it. If they want to shift the status quo by taking unilateral action on their own authority, whether or not that authority is clearly established in law, they can simply do it - quickly, forcefully, and (if they like) with no advance notice. The other branches are then presented with a fait accompli, and it is up to them to respond. If they are unable to respond effectively, or decide not to, presidents win by default. And even if they do respond, which could take years, presidents may still get much of what they want anyway. The bottom line, then, is that the Constitution's incomplete contract sets up a governing structure that virtually invited presidential imperialism. Presidents, especially in modern times, are motivated to seek power. And because the Constitution does not say precisely what the proper boundaries of their power are, and because their hold on the executive functions of government gives them pivotal advantages in the political struggle, they have strong incentives to push for expanded authority by moving into grey areas of the law, asserting their rights, and exercising them, - whether or not other actors, particularly in Congress, happen to agree.
Terry Moe and William Howell, “Unilateral action and presidential power: a theory” (1999)
