The legal questions raised by President Bush's wiretapping seem to me complex, not simple. Here is a rough guide: (1) Did the AUMF authorize his action? (2) If not, does the Constitution give the President inherent authority to do what he did? (3) If the answer to (1) or (2) is yes, does his action violate the Foreign Intelligence Surveillance Act (FISA)? (4) If the answer to (3) is yes, is FISA constitutional, or is it inconsistent with the President's inherent authority? (5) If the answer to (1) or (2) is yes, does the wiretapping nonetheless violate the Fourth Amendment?
I have already suggested that it is plausible to give a "yes" answer to (1), certainly if we do not consider the effect of FISA. It needn't be conclusive that Congress didn't "intend," with the AUMF, to authorize wiretapping. Once the AUMF is in place, the President can certainly engage in surveillance of some kinds, eg, surveillance of Al Qaeda in Afghanistan. It isn't a big stretch to say that he can engage in surveillance of people with known Al Qaeda affiliations who are calling to or from the United States. (If Osama Bin Laden is calling New York, it's clear, I think, that the AUMF allows the President to listen to the call.) If there were doubt about the President's power under the AUMF, a plausible claim of inherent power, under (2), would justify reading the AUMF to allow the President to engage in surveillance. (Of course nothing I have said suggests that under the AUMF, the President can engage in surveillance of people without a tie to organizations or nations associated with the attacks of 9/11.)
What about (2)? The Supreme Court has not decided this question, and some lower courts seem to have ruled in the President's favor on this one. Orin Kerr, at the Volokh Conspiracy, has an excellent post that covers this issue (and others I am discussing here). It is not clear that the President is right on (2), but it isn't clear that he is wrong.
On (3), the question is how to square the AUMF with FISI. It isn't unreasonable to say that the more specific statute, FISA, trumps the more general, so that the wiretapping issue is effectively governed by FISI. But if surveillance is taken to be an ordinary incident of war, and if the President has a plausible claim to inherent authority, this argument is substantially weakened. Note that the President isn't forbidden, by the precedents, from arguing that FISI is unconstitututional insofar as it forbids him from engaging in the relevant activity (item (4) in my catalogue). I am not sure how strong this argument is; if it is pretty strong, there is good reason to read the AUMF to allow the President to wiretap, and not to read FISI so as to forbid wiretapping, simply to avoid the hard constitutional question.
What about the Fourth Amendment? It turns out that the President has a plausible claim here as well (again see Orin Kerr's post for helpful discussion) -- not necessarily decisive, but plausible. The cases do not clearly support the view that when monitoring (a) an international call involving (b) someone with an Al Qaeda connection (c) to or from the United States, the President must, (d) under post-9/11 conditions, obtain a warrant. (The AUMF is helpful to the President here.) But to the extent that the Fourth Amendment claim is strong, there is reason to interpret the AUMF narrowly, so as to avoid that question, and also to interpret FISI broadly, also to avoid the Fourth Amendment question. On the other hand, the President's claim of inherent authority, if plausible, raises doubts about this approach.
This is meant as an exceedingly tentative analysis, with the purposes of disaggregating the issues and of suggesting that there are several unresolved questions here.