Stop talking about "citizens"
Look: any politician who pretends that it's in any way possible to allow powers to be used only against foreigners, not against citizens - is just grandstanding. What they're proposing is unconstitutional in its own right. So saith the 14th amendment.
Either the snooping is legal, or it's not. "Legal, but only against non-citizens" is an idea that is inherently incompatible with American law.
So please stop talking about how these things can be/have been done to citizens. It's a red herring. The question is whether they should be done to anyone.