John Yoo, current law professor at UC Berkley, author of the famous 2002 torture memo and former Bush administration lawyer, has been cited recently as saying "People think that congress has to say OK to everything. But if that were true, that means every war we've fought since WWII has been illegal." (Well, Duh!) He elaborates that Bush's request for a congressional authorization for the use of force in '02 was purely political. It passed overwhelmingly but was not considered a declaration of war. The resolution was only seen as a way to bring democrats on board. (oh, thank you so much for allowing half the country the opportunity to either oppose the terrorists or support Saddam!)
In other words, they don't believe that there is any necessity, except in the political sense, to seek approval from congress for military action.
If you liked Iraq, You'll love Iran!