I’m completely in agreement with Jonathan Bernstein that people tend to vastly overstate the level of presidential power over the legislative process. I think his efforts (here and here) to argue for Presidential weakness in the national security sphere is considerably weaker.
It’s true of course that presidential conduct of foreign policy is constrained by the objective international situation (i.e., Obama can’t make North Korea or China do what he wants) and Presidential power is limited by human weakness (Presidents have only so much time in the day and thus depend on subordinates to actually tell them what’s happening) and by practical politics (Obama can pick whatever fights he wants to with generals, but generals have a lot of prestige in America and doing so might make him unpopular). But it’s still a very different kind of situation vis-a-vis other actors in the system than what you see in the domestic sphere.
Probably the easiest way to see this is to say that if you want to make domestic policy more left-wing than it currently is, what you really need to do is to change congress and especially the Senate—replace some incumbents with people who are more liberal. On national security policy, this really isn’t the case. If you replaced Judd Gregg with Paul Hodes, that wouldn’t change our policy in Afghanistan it wouldn’t alter the kill order on Anwar al-Awlaki, it wouldn’t change the drone strikes in Pakistan, it wouldn’t alter the pace of withdrawal from Iraq, etc. By contrast, if you replaced Barack Obama with someone who had deeply held left-wing views on these subjects things would change. That hypothetical president wouldn’t operate without constraint, but would have substantial latitude to change things especially on the topics he cared most about and was willing to suffer public criticism over.
And for a political activist, this is what matters. Where are the pressure points in the system that make a big difference. On domestic issues at this point that’s congress. On security issues, it’s still the White House.