I would say make 10 accounts, post some very LEFT wing and some very RIGHT wing comments and posts, then add some chatter etc.. then submit these to social bookmarking sites, I would guess if you can get people emotionally riled up then they would register to your site just to debate you... then keep playing both sides with your left and right wing accounts, get more and more people on board, people like to argue that is the benefit of a forum.
Here are some ideas - Advertise on YouTube and Myspace, Facebook ect... - Link Exchange / Banner Exchange - Add a Political Blog and site (See if you can place on a different host , then link exchange amoung them