It will only take one thing: Lack of the possibility. As long as they technologically can, it will happen. They are and always have been above and beyond the law due to their nature of being "secret" services. The very concept of secret service is to get intelligence by any means necessary, as long as you don't blow your cover. Because if you don't blow your cover, nobody will ever know what laws you have broken, so you are essentially above the law.
Be a little realistic, a secret service has and uses back-doors to virtually every known dataset in their own country, and sometimes even from other countries. Google database, Facebook database, passports database, AT&T phone records database, I mean they show this even in the movies, where they kind of tone it down, I image in reality they are not as moral as they want to seem in the movies (as in we can't tap that guy's phone/email/facebook account without a warrant, it's not legal/ethic, yea right). The only true hindrance is that they can't use it in court unless they prove it was obtained under a warrant. But with all the secret courts existence, it's absolutely a breeze to get a back dated warrant.
The technology is here, the power is here, all it takes is the right political context (some big ass fear inducer in the population) and the right leader for the US to become an Orwellian state.