Hey Norman,
I have a Master's degree in political science and I don't think I will have that much advantage over those who don't for a number of reasons. First, there are many different areas to study within the discipline of political science including political theory, the institutions (presidency, congress, courts), political behavior/public opinion, comparative politics, international politics, public law, etc. Some of these topics are relevant, but in the undegraduate classes you don't go into a major analysis and critique of the works. Also, I believe having a liberal arts degree is a double edged sword. In one way it is good because you write very often. However, the writing style is often long and verbose with much extraneous information. My understanding is that if you write like this on a test, you will do bad. Therefore, I think you don't have to worry at all. If you feel that you don't even have a rudimentary understanding of how government operates, I teach political science at a university and would be happy to recommend some good books. You can just pm me.