Fivethirtyeight.com

Not to toot the horn of other political blogs, but fivethirtyeight.com is a pretty good one.  It's operated by Nate Silver (an editor of Baseball Prospectus) and brings a surprisingly and frustratingly rare mathematical and research-based approach to interpreting political polling.  Just as Baseball Prospectus is the best source for baseball information in a world filled with ridiculous conventional wisdoms and opinions based on no research, fivethirtyeight.com is a beacon of saneness in an otherwise confusing and difficult to navigate political world.  I urge you to check it out.