Hashem runs the world. Daven for a president who will show favor to the Jewish people and Eretz Yisrael. As for a particular candidate, it seems inappropriate because we cannot know what is good for us.
Wolf – You’re correct in theory that you can daven for almost anything (although there’s a concept of tefillah shav brought in the ninth perek of Berachos – ie, one hears a scream and davens that it didn’t come from his house. Clearly because this is after the fact, it’s a wasteful tefillah). But, just because you can daven for nearly anything, doesn’t necessarily mean that you should.
Avram – It’s better to daven for foolish things than to not daven at all. Given that Hashem already knows what we want and need, what’s the purpose of davening? It’s for us to realize and appreciate that He is the source of all. Davening for a political candidate is silly given that we don’t know what is good for us, but at least the person standing before Hashem is recognizing or coming to recognize who really runs the world.