I think this would really depend on the area of political science that you are interested in to begin with, because not everything is as conducive to interdisciplinary study. I also think that American Studies people would see through you if you didn't have a genuine interest in doing something American Studies-ish. Am. St. tends to be very different from one school to the next, varying widely depending on whether or not the professors have PhDs in American Studies or the department is made up of people from around campus (i.e., History, Religious Studies, English).
I think American Studies is perhaps another option for some folks, but is not really a back up plan in case you don't get in to a poli sci program. Also, good programs are always hard to get into, even in more obscure disciplines.