This has happened before with Watergate -- and the only reason Nixon got caught was because people talked. I would imagine something similar would happen. Politics can help create some balance here -- if they really were doing something that benefitted one side over another, someone from the other side would likely blow the whistle.
Regardless though, that information wouldn't just be sitting there unencrypted if there was no request look at it: there would be a paper trail that a certain individual was targeted and records pulled. That's not always the case if you just have a huge database of unencrypted communications.
> What if the NSA starts investigating the records of politicians with a anti-NSA stance?
This is a danger I am actually concerned about, but I'm also pretty sure this has been going on for decades. If not with the NSA, then with the FBI (e.g. the Hoover era).
> with power like that, they could basically destroy anyone who opposes them before they even start.
You don't even need government power to do this; the major political parties are set up with this purpose in mind. And work in politics for even a little bit and you'll realize there's not a whole lot of separation between government work and party work. I don't know that this is really much different than the status quo over the last few decades.