Ah... sorry, mate, but I believe it is a Constitutional Republic.
Really? But then again who cares since i don´t live there ;-) But jokes aside! Fact is when i think about the US i think about a democratic country (It´s not so important for me how the exact form of definition is).
But i admit you are correct.
Cia world factbook says: Constitution-based federal republic; strong democratic tradition
greetings