He won't get Congressional approval, and when he doesn't get it, he won't strike on his own. If he does he'll almost certainly be impeached. He pushed the Constitutional envelope pretty far with Libya as it was, but if he asks for and doesn't get Congressional approval, and then goes ahead and strikes anyway, he's toast.
During the 2008 campaign, Obama declared, "The president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation."
With Libya he really only had a day or two to put together a strike (along with European and Arab allies) to prevent a possible slaughter of Libyan civilians. In the interest of expediency in saving the lives of Libyan civilians, Obama sidestepped his 2008 statement, put that particular principle on hold, and even though the strike was understandable, he took a hit politically. But the Syrian situation, at this point, isn't even understandable.
He should have just gone ahead and done it within a few days of the chemical attack and said the military response was to prevent another imminent chemical attack. People, including our frenemies abroad, could have bought into that. But now he's red-lined himself into a corner, the time has passed for unilateral action on his part, and he has no choice but to ask Congress for approval, lest he be viewed by others around the world as even more of a weak
wimpling than he already is. He can't get other countries to back his play, either. He was like 1-19 at the G20 Summit, with that perennial powerhouse France being the only one to stand by him in support of a military strike. The surest sign that I know of that you're doing something wrong is when France agrees with it, and when they're the
only one who agrees, it's a lock.
Yes, mass destructive chemical weapons were used and that's bad, bad, bad. A response is warranted. But there are a million responses that can be done before a military strike is the only one left to try. If he doesn't get Congressional approval and he goes ahead with a military response anyway, he'll be viewed both home and abroad as being out of control, and dangerous. Our enemies and frenemies will exploit that. What's going on in Syria is, in the end, a civil war in a sovereign nation.