At least George Bush declared war using constitutional methods. The Constitution authorizes Congress to declare war in Article I, Section 8:
The Congress shall have Power to...To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and WaterArticle II, which lists executive powers, does not even mention the word 'war'.
Under Bush, Congress voted for the Iraq invasion. Obama announced his intentions from Brazil, without so much as an afterthought of congressional approval. Whatever you think of Bush's actions, Obama's are like his, except on steroids.
Code Pink's unequal treatment of Bush and Obama's military actions show that this isn't about policy or constitutional legitimacy of war, but a chance to campaign against a Republican president.