“If undergraduate programs are not rated, then they are simply . . . not rated.”
merc81, graduate school rankings are a legitimate indictor of the quality of the department, regardless of the level. As you point out, LACs and universities with low research activity may offer absolutely stellar undergraduate instruction in several disciplines at the undergraduate level, and that is not in dispute. But I think it is fair to say that if department is strong at the graduate level, it is also going to be strong at the undergraduate level. Why wouldn’t be? You will essentially have the same faculty teaching the same material (albeit watered down and less statistical) in the same facilities.
The issue I have with lumping International Relations with Political Science is that IR is merely a sub-speciality of Political Science. As I said above, if the OP wishes to study IR, which he/she did not specify, then the DC schools are great. But if she/he wishes to focus on the broader spectrum of Political Science sub-specialities, they may not be as good as other departments around the nation. My point is, why limit yourself to just one city when there are many other excellent programs around the nation?
“Rated by whom? And on what level?”
Take your pick. Any ranking of Political Science or Psychology departments will look roughly the same. The two main sources of US programs and departments that I can think of are US News graduate school rankings and the National Research Council.