Mixing family and politics?

What is a mature/appropriate way to handle political disagreements within extended family? For example, at family gatherings, etc., a sister or brother-in-law might start talking positively about their party or candidate while putting down yours. What's the mature/appropriate way of handling people like that? Does it matter whose house it's at? I mean, if it's at their house, must you be respectful and put up with the perceived abuse? Or is it appropriate to give your opinion - even though it's going to offend them? How do you handle these types of issues?