The Debate Scenario...
Even before Congress takes a vote, concessions are already being made by Russia and Syria. They haven't offered to give up Edward Snowden yet, but who knows?
It is very possible the Congress will vote down the resolution. And some Republicans will threaten to impeach the President if he goes ahead and bombs Syria anyway.
The President will simply say that he has the authority to defend our country if our national security is threatened and that he will not be deterred by the Congress.
Negotiations with the Russians and Syrians will continue in the back channels. The US will attempt to get more concessions from the Syrians. They will ask that Assad step down and that a successor be named until elections can be held.
The Republicans will continue to whine and blame the President for weakness as this scenario proceeds.
In the end, the President will get what he wants from the Syrians without any missiles being fired - unless, of course - Hezbollah decides to lob a few missiles across the borders. Then Iran becomes the new target.