I think that it is great that you have decided what you want to do. Some of us take a little bit of time for these things. I think the thing that you are missing is work experience. I know that, at least from what I've heard, is that Columbia likes work experience. It seems that you would need a couple of years of work experience. Also, a lot of grad schools allows space for you to comment on why you didn't perform as well academically -- that is really important. You said that you were majoring in political science -- I know that MPP programs depend a LOT on quantitative experience, I would suggest, if you can, minoring in economics.
Good Luck!