I think there is something deeper to this. Even with no evidence that the NSA is spying on congress, he can still put them on a very difficult dilemma. The problem is as follows. If the NSA says yes, then they will admit to spying on the rulers of the US and opening up the possibility of blackmail implications, but best of all turning Congress against the NSA. If the NSA says no, then he can ask the NSA why they spying on the American public and not Congress when no laws should be applied differently.
My guess is that the NSA will reply something like, "We do not separate data between regular citizens nor congress men, but we further do not actively seek out data on any congressman or government official."