From Andrew Sullivan's site, which I subscribe to. First he quotes a Washington Post/ABC News poll:

Only 32 percent said Obama had explained clearly why the U.S. should launch strikes. Back in March 2003, as the Iraq War started, 49 percent said that President George W. Bush had compellingly made his case for what was then at stake.

Then he adds this:

So Obama has much less domestic support than Bush, no backing from the Brits, open hostility by the UN for immediate war, and an obviously conflicted administration. This is a war even less likely to succeed than Iraq and even less popular.

The war in Iraq didn't fail because of an erosion in domestic support; it failed for other reasons that were reflected in an erosion of domestic support. 

This is not an argument in favor of missile strikes in Syria, by the way. I don't know enough on that subject to even begin discussing it.