Honestly, just tell her you'd rather not talk about politics. It can be incredibly passive. You do not need to elaborate. You can make up something simple like "I just want to focus on work while I'm at work," it doesn't even need to make sense. If she pushes back, drop it, but don't ever say it's okay for her to do. Then if she keeps doing it talk to your manager about it. "I've talked to her and asked her not to talk about politics, but she insists. I find it distracting." Again, it doesn't necessarily need to make sense and it doesn't need to be over the top. Now,. hopefully your manager will sort it out. Because if it's annoying you it's likely annoying others.