I think there are 5 key ingredients in order to be a successful political blogger. In order of importance;
Focus/Subject Matter Expertise - Matt Wardman has suggested modelling yourself on a backbench MP having two or three "bee in your bonnet" issues. Whilst breadth of posts can be a good thing, there should be certain issues on which you can take the role of authority. If you can't demonstrate some expertise or unique insight, building a readership will be like trying to roll a snowman in the Sahara.
PR - Because it's the internet your site has to make an immediate impact so that visitors know within seconds what your blog is all about. Guido manages this. You should also be aware of the importance of exchanging links and commenting on other blogs. An interesting ID/pseudonym (like Praguetory or Peter Hitchens) attracts traffic, too.
Time - What you see on a blog is like an iceberg. A great deal of research, networking and planning goes on behind the scenes. If you don't have a great deal of time, you need to think carefully about what aims having a blog could achieve.
Psychology/Arguing - The moment you set up a political website you will have created legions of instant supporters/enemies (particularly if you choose a provocative blog name). You need to be able to handle criticism warranted and otherwise and should spend some time honing debating skills so that you are aware of how to argue effectively online.
Writing - It's easy to carried away when you're writing about a subject that interests you. Be aware of the readability of what you have written. It's an unfortunate correlation that the more difficult something is to read the more respected the commenter is, but I still think you should tend towards conversational writing when blogging.