It is very possible that the president will not obtain a joint authorization to bomb Syria; if he chooses to go ahead and attack anyway, Obama will incite a constitutional crisis—the first time in history that a president has decided to go to war against the declared wishes of Congress. The public and the courts will adjudicate the legality of that act, and it would be contentious.
So the corner that Obama has painted himself into is now inescapable. Defying Congress will put the country into a Watergate/Monicagate mess. Not doing anything will confirm the administration’s impotence and only enhance Russia, Iran, Assad, China, Islamists, and almost anyone else who does not like the U.S. Doing something small, with or without congressional approval, will be looked upon as a cynical waste of human lives to restore Obama’s credibility, the sort of craven, immoral political act that a younger Obama made a career out of mocking. Doing something big will invite public and global outrage if only moderately successful, and doom the Obama presidency if unsuccessful.