From what I can tell, to the extent that he's being honest and not muddling the issues, Posner is arguing only that probable cause is too restrictive; he's not presenting any arguments about other parts of the program. For example, I don't see any arguments against a hypothetical law that would allow the FISC (also called the FISA court) to grant warrants so that the NSA could search people when it merely had "reasonable suspicion" or "reasonable basis to believe" that they were associated with terrorists. Bush's current program, as far as I can tell, and as far as Posner has actually argued, has no advantages whatsoever over such a law.
[Glenn Greenwald's analysis of the administration's 2002 statements has been discussed extensively on this blog already, so I'll skip part of my post and assume you know about it already]
Regardless of what [the Baker revelations] say about the administration's honesty, [they mean] mean that [Posner's] argument that probable cause is hampering investigations is patent nonsense. With all due respect to Posner, Baker simply knows more about the issue than he does, and every argument the administration has advanced ("FISA is too slow," "probable cause is too restrictive," etc.) has been buried. The only possible reason for operating outside of FISA instead of amending it is to avoid court oversight. They're doing some kind of spying that the courts wouldn't allow even if the standard were lowered to reasonable suspicion.
We know that the administration has illegally spied on its political opponents, including Democrats, anti-war protesters, and Quakers using other programs. It's either that or something much much broader than what "reasonable suspicion" could possibly allow--monitoring a significant fraction of all the calls in the U.S.
(This "narrow" or limited aspect of his methodology, in contrast to his methodology in its entirety, which would require a review of his larger corpus or would at least need a broader statement from him.)
I'm done here, am beginning to repeat some things.
I have focused instead on the constitutional authorities of the President because I believe that those authorities should be the beginning and end of our legislative inquiry into the "legality" of this program. It is quite clear to me that Congress could not, through passage of FISA, extinguish the President's constitutional authority to conduct the terrorist surveillance program at issue.
But, unlike the authoritiy to seize steel mills -- by comparison so indirectly and distantly tied to the President's Article II authority and so directly tied to Congress' enumerated authorities in Article I -- the regulation of the President's constitutional authority to collect intelligence information incident to potential or actual attack by foreign powers and their agents is a subject over which Congress cannot assert complete dominion.