However, it's inconsistent of Paul's defenders hide behind the "Congress should declare war" spin to justify his vote against the war in Iraq since he did vote to authorize military force in Afghanistan without a declaration of war.
Actually, he didn't. The text does not specify "Afghanistan".
Ron Paul voted for the AUMF against the 9-11 terrorists because "To declare war against a group that is not a country makes the clear declaration of war more complex.... The proposed resolution is the only option we're offered and doing nothing is unthinkable." Ron Paul, http://www.house.gov/paul/tst/tst2001/tst091701.htm
However, while it may not be possible to declare war against "a group that is not a country", Iraq is a Country -- and the Constitution provides clear details on how the United States is to go about making War on a Country. Specifically, Congress is to issue a Declaration of War, as Ron Paul advocated.