This has never been a 4th amendment issue, but a national security issue. The courts have consistently supported the President's right to collect intelligence for national security purposes without a warrant. The only time a warrant is required is if a criminal prosecution is the goal. The FISA court was set up as a mechanism to allow information gathered through intelligence resources to be shared with law enforcment by setting up a "secure" method of obtaining a warrant.
I think Brownback is trying to determine if any Fourth Amendment issues were violated. If he is then he's on the right track.
The 4th amendment and national security are in tension. The term of art where the president has the most leeway for warrantless surveillance is not "national security purposes," but "Foreign intelligence information."
The only time a warrant is required is if a criminal prosecution is the goal.
There are exceptions to a requirement for a warrant there too, but in general I agree - there is tension between criminal prosecution and warrantless surveillance.
The FISA court was set up as a mechanism to allow information gathered through intelligence resources to be shared with law enforcment by setting up a "secure" method of obtaining a warrant.
I think the original function of FISA as a whole was to provide various degrees of oversight and separation "between" foreign intelligence and domestic law enforcement. The FISA review court's opinion in In Re: Sealed Case No. 02-001, 310 F.3d 717 (Foreign Int. Surv. Ct. Rev. 2002) provides a good history of how it sees the interplay between inherent powers of the executive, FISA statutes, and barriers to sharing information between various agents within the executive branch of government.
Although it's not exactly certain what the NSA was doing, I think it's generally acknowledged on all sides that FISA was bypassed.