Basically, there are Senators--as we all know--that are for the AWB.
President Obama not taking on any sort of Firearm Politics at this juncture of the campaign season makes sense. I will point out that the AWB was under Clinton, he signed it, and he got a second term. I am of the opinion that relating the Democrats major loss post AWB gives much credit where it's not due. Democrats didn't lose primarily because of the AWB, IMO.
If anything there are man anti-firearm types that would be more than happy to get to the polls to vote for President Obama, if he came out and stated that he was seeking to push such a policy. He wouldn't though, since the House is an insurmountable hurdle, and the Senate has a majority of Democrats, and Republicans who have no interest in pushing that sort of an agenda.