Positive and negative approval
Your contrast between the way liberals view majority rule with respect to the First vs. Second Amendment makes me think about the way Obama has been trying to work up an atmosphere of "majority rule" in the first place. As Elizabeth noted, it's a load of malarkey; 90% of the public most certainly does not agree with the President's position. 90% of the public does not agree about much of anything, a state of affairs I find absolutely glorious. The Left praises, and seeks to manufacture, unity; give me beautiful, sloppy, creative, competitive disagreement any day of the week. Somehow the contentious and competitive always end up feeding and clothing the "unified."
The way liberals try to manufacture these "unity" statistics is to cultivate what might be thought of as negative approval. In other words, they throw some general idea - usually very poorly described, and always presented without reference to its drawbacks - in front of a focus group or polling sample, in an effort to get some useful percentage to allow that it sounds kinda sorta okay. Usually the idea is couched in terms that make it sound like only a heartless or unreasonable person would disagree.
But establishing that some proposal, such as universal gun background checks, sounds relatively unobjectionable is a far cry from securing positive public approval: "Yes, let's do this now, and make it a priority." That's the difference between polls that show some percentage of the public thinks the background check idea sounds acceptable, and polls showing an absolutely miniscule number of people who think it's something Congress should be spending time on.
Also, once the discussion moves from "Do you approve of this general idea?" to "Do you think we should do this right now?" people start thinking about the drawbacks, shortcomings, trade-offs, and objections raised by people who care about the issue more than they do. Many Americans do not own a gun, and have no near-term plans to purchase one. When the time comes to secure positive approval for gun control measures, many of these non-gun-owners listen to what gun owners think, and make entirely reasonable allowances for their direct interest in the issue.
I also suspect the increasingly tenuous connection between the Democrats' grab bag of gun-control measures and what actually happened in Newtown had a lot to do with soft public support, once it was time to move from soft approval ("We've got to Do Something!") to specific legislation ("Let's do this!") People who might have generally found the background check idea reasonable also couldn't help but notice it had nothing whatsoever to do with the Newtown shootings... and no matter how much Democrats love their little episodes of political theater, a lot of people just don't like watching politicians exploit tragedy.