First, to say that the presidency is Constitutionally weak is certainly not to say that it is powerless. To the contrary: presidents have more potential to influence American policy than any other single person. But that potential is still limited; they can't just do whatever they want.
Second, institutional constraints caused by Congress are perhaps the most visible and obvious ones, but they are not the only, and sometimes the most important, ones. Serwer talks about "serious institutional blocks represented by say, a GOP filibuster," and yes, those are important. But so are other sorts of obstacles: from bureaucrats, from interest groups, from the president's party. One of my favorite examples is DADT, in 1993. Yes, Bill Clinton could have simply ordered the ban on gays in the military out of existence. In theory. In reality, the strong resistance from the military defeated his original plans. Or, take one of Fred Kaplan's favorite topics, the division of the defense budget between the services. Again, presidents may want to change that, but they're unlikely to succeed. It's important, in my view, to take those as seriously as we take resistance from Congress (and often resistance in Congress as actually a manifestation of blocks from elsewhere in the political system).
Third is the question of the relative position of the president in domestic and foreign affairs. My position? It's complicated! As I've said, the president does have an advantage in foreign affairs in that his formal powers as commander in chief strengthen his hand against an important portion of the bureaucracy (that is, the military). But there are all sorts of things in various different arenas that add or subtract from the president's formal powers. For example, in dealing with the economy, he is limited by the independent Fed; that's a big deal. On the other hand, in foreign affairs he has to deal with, well, foreigners -- not just enemies, but allies who sometimes have very different interests at stake in a particular arena. One of the president's biggest advantages in foreign affairs is that in many cases, very few US citizens care very much about what happens. When that's not true (think Israel, or Cuba), the president's options are far more constrained. Of course, that works both ways: when the president doesn't have an incentive to care very much, he's more likely to be rolled by smaller interest groups or by the bureaucracy.
Fourth, both Serwer and Lemieux complain that, in Leimeux's words, my point about whether president's were actually the authors or even supporters of the policies we attribute to them
gets to be like speculation about whether John McCain is “really” an anti-abortion zealot or whether George Wallace was “really” a racist. At some point, for public officials, the proof of the pudding is in the eating.Serwer calls the same thing "dangerously close to existential questions about whether any of us truly have free will."
OK, let me try to tackle this a bit. First, I agree with Lemieux that what matters is what pols actually do, not what they "really" believe. However, it's worth keeping in mind a couple of things...first, we don't necessarily see everything they actually do. Some fights don't happen in public. And we also know that there are strong incentives for pols to support the public side of what they've done, regardless of whether they actually supported it in behind closed doors or not. I'm not talking about what they "really" think; I'm talking about what they actually acted on. We might not know the answer to that, especially on national security questions, for some time (and perhaps we'll never really have good evidence). But we have plenty of history that shows presidents, like all pols, sometimes fight for one thing in private, lose, and then support the opposite position in public. What's more, support or opposition aren't binary. Presidents are constantly faced with multiple battles on multiple fronts, and they don't only pick and choose "sides" in a dispute; they also pick and choose whether to fight at all, and if so how hard.
One important point is that in sorting all of this out, analysts and activists have somewhat different jobs. For an analyst -- and in my view, this is as true for reporters or bloggers as it is for academics -- sometimes the correct answer is: we don't know. We may have to wait until more interviews are conducted, more books are written, more memos are declassified. Of course, we're not all in a position where waiting passively is the only option (some of us can choose to seek those interviews), but until the data are available, we should admit that we really don't know. Activists, of course, cannot do that. For them, it may make perfect sense to put pressure on the president, and not worry about whether he's actually working on their behalf behind closed doors or not. If he's not, outside pressure might get him to switch sides; if he is, perhaps outside pressure will influence him to elevate this issue over some other issues (that is, get him to fight harder). Even so, activists are better off understanding these distinctions, because without them one might easily wind up concluding that no differences exist between pols in cases when, in fact, very important differences do exist.
OK, I think, perhaps, that covers some of the main issues. I went light on the examples, because this is way long...it seems that I'm going to be on this topic for a while, however, so maybe next time I'll be less abstract. And for regular readers wondering about it, yes, the long-in-progress post about secrecy and presidents (or something like that) is still in progress.