I started working at this company 2 months ago and my boss has an offensive politically focused sticker on his office door. Business ethics wise, I don't think that political opinions have a place at work. Plus, he is the only on in the office of 80 people who has a political sticker on his door. Furthermore, if I were to play devil's advocate and put up the opposite political argument in the form of a sticker on my door, I would be fired or asked to take it down. How do I ask HR that it be taken down? Should I contact HR anonymously? I'm certain others feel the same way, however, no one has come forward. What should I do?

You should do nothing because you are a newcomer and learning the lay of the land, and the ways of the office politics. Why stick your neck out, when you should be fitting in? There will be more facts later, so for now put aside your own politics, and pay attention.

Pretend you're a parent, and your child comes home and tells you that the kids in school are calling him names. What would you advise your child to do? Most likely you'd say what all parents have said since the first cave man kid was picked on, which is to ignore them, words can't hurt you.

This is the same thing. It's a sticker, and it can't hurt you, so ignore it. It won't effect your work, and it doesn't effect his, so reporting it will look like nitpicking. They'll wonder why you have so much time on your hands that you go around noticing stickers on doors, and worrying about them, instead of focusing on your job.

It's really a non issue.

Work your way past him in rank and then you can order him to take it down.
Can't wait that long? Rip it off when no one is looking, and hope there aren't any hidden cameras.
Or paste a Hello Kitty sticker over it.
Doesn't he have a boss? If that person has seen it and done nothing, you do nothing.
I think you think that the workplace follows nice neat rules. It doesn't. If you aren't top dog, you take your lumps.