To get started, head to facebook.com/ads/preferences. Here, you’ll find a large collection of “interests” Facebook thinks you have, sorted into categories. Click on “Lifestyle and Culture” to find, among other things, where you land politically. If you haven’t explicitly Liked the Facebook page of a particular politician, Facebook will guess and place that guess here.
The entire ad preferences page is a fascinating look into how Facebook analyzes and categorizes its users. If you don’t want a particular topic influencing the ads you see, you can remove it here. Obviously, you can’t turn it off entirely, but you can tweak it.