In light of what we do know about the NSA, it is completely reasonable to assume (and in fact believe) the worst when we speculate on what we don't know. If people ask, "what evidence do you have?" I just respond with, "I'm sorry, but have you actually been paying any attention to Snowden's revelations and have you done any critical thinking about what they actually mean?" That is enough to shut them up.
Power corrupts, and absolute power corrupts absolutely. And the NSA has as close to absolute power as one can reasonably reach today: in the Age of Information, they have access to almost all information. What makes them especially dangerous is that their deeds are powered by their misguided conscience. They strongly believe that what they are doing is the Right Thing because it protects America. People who have succumbed to such fundamentalist thinking will do anything with the power they have, including blackmailing politicians.