How to stick up for what you believe politically?

I know this sounds dumb, but I freeze up when I try to stick up for my beliefs, especially when I am getting ragged on by this one person for what I support (politically) at school. Instead of standing up for myself, I stand there, speechless, letting him call me "uneducated" and "narrow-minded" or just making snide comments about my political beliefs. It is so annoying. I know if I say anything I will just stir more trouble up or I will sound completely stupid. This makes me feel even less confident than I am now and that I really am stupid. Can anyone help me out?