The Manchin-Heitkamp resolution* calls for two primary things. First, it gives the administration 45 days to secure from Syrian President Bashar al-Assad a commitment to join those nations who have signed and agreed to the Chemical Weapons Convention. If Assad fails to comply, then the Senate gives full authorization to the president to use whatever means possible to respond to the regime's apparent August 21 use of chemical weapons.So now, thanks to the U.S.-Russian agreement on a Framework for the Elimination of Syrian Chemical Weapons, Assad has already acceded to the Chemical Weapons Convention. And he will be required to provide an inventory of Syria's CW arsenal within seven days of ratification by the Organization for the Prohibition of Chemical Weapons of a draft decision to be provided by the U.S. and Russia "within a few days." Once that inventory is provided, the Chemical Weapons Convention forbids moving any weapons, "except to a chemical weapons destruction facility" (Article IV, paragraph 4). Inspections are to begin in November, and the framework, as the FT puts it, "is structured around a series of deadlines which will allow the world to judge whether it is being adhered to or not." The framework also stipulates enforcement under Chapter 7 of the U.N. Charter, which authorizes the Security Council to use both non-military and military means.
What the agreement (already famously) does not do is effectively pre-authorize military action in the event that Syria violates the agreement. Whether this kind of agreement could be so structured, I don't know, but the Russians have made it clear that they will block any future call for a Security Council resolution authorizing force.
And that's where, in a sane U.S. political environment, a revised Manchin-Heitkamp AUMF could fill the void. The U.S. has reserved the right to use force as it sees fit. Legislation authorizing the President to do so if the Assad regime uses chemical weapons again would provide an enforcement mechanism.
An AUMF of this kind would preserve what I see as the agreement's primary purpose (from a U.S. point of view): to prevent the Assad regime's further use of chemical weapons. Securing and destroying the Assad arsenal will be a long, elusive, frustrating, unpredictable process, as was containing and ultimately eliminating (had we only known...!) Saddam's stockpile. But preauthorization from Congress in Obama's pocket would be almost as effective a deterrent to further CW use as a likelihood of Security Council action.
Of course, getting such a resolution passed would require almost all Democrats and a sizable minority of Republicans to trust Obama enough -- or, in the case of the GOP, to stop pretending to mistrust Obama enough -- not to drag the U.S. into another war or trigger a further spread of chaos in the middle east. In a rational political environment, it would seem to me that Obama has negatively earned that trust by proving himself highly reluctant to strike Syria. Moreover, Congress would only be authorizing what Obama already has the power to do, and might have done two weeks ago. But a rational political environment is not the one we live in.
*Manchin-Hekitkamp also calls for the administration to "submit a long term strategy for Syria" and slants the imagined plan toward providing material aid to the Syrian military and political opposition.