Next, the common argument is that the president has breached his duty to uphold the constitution (and therefore, we should impeach him!). Under a recent SCOTUS decision, Hamdi v. Rumsfeld, the Court recognized that the congressional authorization of force for both the Afghanistan and Iraq conflicts conferred on the executive branch the same powers as if there was a formal declaration of war. This is important because for nearly 200 years it was understood by all three branches that intelligence collection--especially in wartime--was an exclusive presidential prerogative vested in the president by Article II, Section 1 of the Constitution. Washington, Madison, Jefferson, Hamilton, John Marshall and many others recognized that the grant of "executive power" to the president included control over intelligence gathering. In fact, in Federalist 64, John Jay (not a bad jurist in his own right) praises the constitution for giving the executive the ability to "manage the business of intelligence in such a manner as prudence may suggest." In recent administrations, former Attorney General Griffin Bell and the now-infamous Jamie Gorelick have agreed that inherent executive powers include power over intelligence gathering. Courts also have agreed that warrantless searches for foriegn intelligence are lawful provided that the primary purpose is for foreign intelligence-gathering and not for law enforcement. Also, purely domestic communications cannot be targeted (Legally, this seems to be the reason they would filter out non-essential data and data dealing primary with U.S. targets). Because this program is reviewed every 45 days and activities must be reported to congress, there are sufficient checks on it, and I have no doubt that it is legal under the current definition of "inherent" executive powers.
The Constitution should be the final say (because Congress cannot take away powers from the executive -- even if the New York Times hates him), but for those who do not believe in "inherent" executive powers, there is evidence in FISA itself that the president had the authority to authorize this program (as an aside, and I REALLY hate this argument, but for those who are interested, he was not the first president to do this, or to think that the president had such "inherent" powers). Section 1802 of FISA is entitled, Electronic Surveillance Authorization Without Court Order and gives the president power to conduct such surveillance (surprise!) WITHOUT A COURT ORDER. The short of the argument is that a warrant is not needed for communications between a foreign power and its agents. A foreign power is defined within the act as a "group engaged in international terrorism or activities in preparation therefore." The argument is a little more complicated than that, but it is definitely there (and space is short here).
The real kicker is that the act requires the attorney general to report to the House Permanent Select Committee on Intelligence and the Senate Select Committee on Intelligence 30 days prior to the surveillance, except in cases of emergency, when he must report immediately. He must furthermore "fully inform" those committees on a semiannual basis thereafter pursuant to section 1808(a). This is significant, because it means that some of the same politicians who have been charging abuse of power may also have been briefed on what was going on long ago.
Also, there are many examples of searches conducted absent a warrant that are perfectly legal under the fourth amendment. I would guess that the well-excepted exigent circumstances exception/rationale applied to many of the instances where this program was invoked. Further, the question could become whether the fourth amendment acts as an internal limitation on the powers of the executive. I happen to believe that it does not with this particular power because of the limitation (must not be for law enforcement) and the nature of the issue (intelligence/national security ABROAD). The distinction between domestic affairs and security threats abroad is important because the president is the figurehead of the branch that deals with international affairs, and it is unlikely that an amendment protecting our citizens' privacy interests would act as a limitation on the powers needed to deal with foreign affairs essential to national security (those held by the executive).
There are two ways of looking at this. Either the grant of executive power allows this program (and nothing FISA does can limit this because congress cannot re-allocate the powers of the executive), or FISA is granting a power (explained through the statute itself as I showed a glimpse of above) constrained only by the fourth amendment (where the legality would be determined, ultimately, by the Court).
The truth about the NSA program is that only international communications are targeted, and the government must have information that establishes a clear link to al Qaeda or other terrorist networks before interceptions are made. As the president said the other day, "if you are talking to Al Qaida, we would like to know why."