I don't know why the obvious hasn't been spelled out. By punting his decision to Congress, President Obama is setting up the Republicans to take the blame whatever is decided. If Congress, which is in the hands of the GOP, votes to attack, he will lay the blame for whatever goes wrong on the Republican congress. If Congress gives the red light, Obama can lay claim that he wanted to attack but the GOP controlled congress said no.
Of course, that will manifest into further proof that the GOP cares more for it's hatred of the Black President than it does for the safety of the country,
By waiting a week/ten days, the memory of wait Assad did will fade among the low-info voters and Obama will be made to look decisive by his sycophants in the media.