I think history & econ can make a great background for polisci.
If you've never taken a class in this area, or haven't in a long while, maybe read an introductory textbook on IR or International Politics to familiarize yourself with the terminology, etc. Show your knowledge in your SoP. If you can, perhaps try to do something to show your dedication to the field, like maybe working as a research assistant (this could also get you an LoR from someone in the field, which helps). This isn't my subfield so I'm afraid I can't be of much help... good luck!