There's a lot of choices online and the best political forums are those where you feel at ease knowing you could express your opinions and can become involved in constructive arguments with your fellow members without being personally attacked. The forum will have an active membership with regular new posts and people available for discussion. There will probably be a variety of new threads opened at least on a weekly basis. Politics is a fast moving area and nobody wants to belong to a forum, political or otherwise that nobody posts in.
When you join a new membership site always be sure you learn the rules and observe them. Otherwise you can find your own posts being deleted and in severe instances you could be requested to leave. Have a read through the posts to see whether or not the threads become an interesting discussion or simply an argument - the online equivalent of a shouting match. Political forums are by definition a place that people would go to argue. So it's good not to be overly sensitive about the replies one of your posts could get. However, if the individual responding has launched a personal attack on you instead of entering into a positive argument, you should not respond in kind. It's ideal to inform a moderator or the community forum owner who can handle the problem.
Politics covers such a diverse variety of topics that it is possible to be a Democrat yet have really different opinions on a particular situation to a fellow Democrat. Fortunately we live in a society that promotes freedom of speech and gives you a chance to debate your very own view without fear of punishment. But sometimes people take advantage of this freedom specifically in internet political forums where they can register without exposing their real identity. They may hide behind their call handle and employ the membership webpage as a way to vent extreme ideas. A well moderated forum will often do their best to stamp out this kind of behavior.
