The Guardian poll you cite is from 18 months ago, and it was just one poll conducted in the midst of a war zone. A slim majority of those polled preferred that Assad stay in power, but of those polled, half of them want him to quickly pave the way for political parties and free elections, and Assad's only purpose would be to prevent a civil war in the interim. Maybe Assad didn't like his poll numbers and decided to gas his people back to their senses. As for Obama's approval ratings, I'm sure they'll spike again as soon as the next Republican presidential hopeful says something outlandishly crazy or insensitive and more people remember why they voted for Obama in the first place.
War often increases a leader's popularity.
Particularly if the country is winning.