There are many points of constitutional law that are unsettled or debatable, including in the area of war powers, but one thing is certainly clear: The Constitution vests the sole power to declare war in the Congress. Period.
This means - and the Supreme Court has held - that this sole power to declare war also vests Congress with the sole power to circumscribe the war that can be fought. And, while a minority of scholars may dispute it, most agree that this also means that Congress has the sole power to declare a war over.
None of this depends upon the War Powers Act, or even the Iraq war resolution enacted pursuant to it. Congress's power to declare - and to "un-declare" - war derives entirely from Article I of the Constitution, and is plenary. Congress can declare the US at war, can delimit the extent of the war and the means to be utilized in its pursuit, and can declare it over, all through solely congressional action - presidential veto is not involved at all. The President cannot "veto" a war resolution by Congress, he cannot veto the conditions Congress places on the war's conduct, and he cannot veto a congressional declaration ending the war.
The enactment process for legislation spelled out in the Constitution, involving presidential veto, is not implicated in such instances at all: This does not involve enacting legislation, but declaring and un declaring war, which is a separate area of congressional power entrusted solely to Congress by a separate constitutional clause.
The War Powers Act was enacted in the wake of the Vietnam War as a way to codify a more detailed understanding of the war powers spelled out in the Constitution. As is the case in many areas of government, we do not have a complete "separation of powers," but an intermixing.
In the area of war, while Congress has the sole power to declare hostilities on or off, the branches jointly share authority for treaty-making - either to end hostilities or to avert them before they even start - and the Constitution makes the President commander-in-chief of the armed forces. This latter fact has led some to argue that presidents have an independent war-making authority of their own. And, clearly, a president can order American arms into action as needed, without prior consultation with Congress, to repel an attack on the United States, its citizens, or its vital interests.
Where the line falls between actions a president must take to defend the country or its troops without congressional sanction, and Congress's clear constitutional authority to commit the nation to war or not, has been a subject of intense, on-going, and unsettled debate.
The War Powers Act attempts to fill that void, by laying out a process whereby the president can act to defend the country where there is no time for congressional consultation or action, but otherwise requiring the president to gain congressional assent to military action before-the-fact where possible or within a reasonable time after-the-fact, where necessary. But nothing in the Act - whether or not it is constitutional - can disturb the underlying constitutional commitment to Congress of the sole authority to take the nation into or out of a war.
In the case of Iraq, the President sought and obtained a congressional authorization under the War Powers Act. That unilateral authorization by Congress can be just as unilaterally recalled by the Congress and the war de-authorized. In addition, Congress defined the terms of the president's authorization to make war in the Iraq war resolution and, if the president has exceeded his authority in the conduct of that war, then he cannot legally maintain his actions - no further congressional action is needed to make that so. A de-authorization vote, then, would essentially constitute a statement that, in Congress's view, the existing war in Iraq was not authorized under the original 2002 resolution, based upon the President's claim that Saddam was a threat and that he possessed WMD. That war, and its lawful authorization, already ended with the removal of Saddam and the ascertainment that Iraq had no WMD. In short, the current war is a war without legal warrant and if Congress so declares the President cannot legally continue to conduct it. This, again, does not constitute legislation subject to presidential veto but flows directly from Congress's powers under both the War Powers Act and the Constitution. Finally, regardless of the WPA and the 2002 Iraq war resolution, Congress can, under its sole constitutional authority over war, declare the Iraq war to be over. As stated above, the President has no power to veto such a declaration, and no law other than the Constitution itself is required to be enacted to give the declaration effect - so, the presidential veto power over legislation simply is not implicated at all here.
Nonetheless, it is important to understand that de-authorization of the Iraq war would almost certainly produce a legal and constitutional stand-off between the President and the Congress, which ultimately would have to be decided by the courts. I have no doubt that the courts would uphold the Framer's original intent in vesting Congress with the sole authority to declare and limit the nation's war-making. But regardless of the legal arguments the Bush Administration may try to mount in response, a de-authorization vote undoubtedly would greatly increase the pressure on President Bush to end the war, and that alone is a step forward.
The point here, is that we have to try everything to end the war. Bill Richardson has more foreign policy and diplomatic experience than any of the other candidates, and he alone is calling for deauthorization of the war before Congress recesses for the summer.