Keep in mind I'm not saying they don't want to get rid of all guns; I'm saying they can't do it immediately, right after Obama takes office (assuming that happens). Even your comment about the Supreme Court justices argues my point. It's not like several justices will retire in January.
And no, they cannot do "anything" they want. There is absolutely no precedent or mechanism for an executive order that would completely nullify a provision of the Bill of Rights. It simply can't happen. The only way a total ban could ever happen, absent of a "clear, present, and immediate danger," would be by legislative action. And there's no way the public, the legislature, or the court would stand for the argument that 30,000 people getting killed by guns annually constitutes such a threat. Obama wouldn't even consider positing such an argument. They can argue that the framers of the Constitution didn't have "assault weapons" or handguns in mind when they wrote the Second Amendment, but how can they argue, now that the Supreme Court ruled that the right is an individual right, that total disarming doesn't violate the Second Amendment? It's always been about how far they can go, not about whether they can ban all guns. They would have to repeal the Second Amendment, and you cannot do that by executive order. And it's not like every Democrat would vote for it. The party leadership, and a majority of democrats, are anti gun, but it's far from all of them. Just look at the NRA ratings. You'll see plenty of Democrats in there who are graded "C" or above.
While I agree that there's the will in the Democratic party to ban all guns, I don't think there are the votes, at least in the House. People will take small, incremental laws like magazine, assault weapon, and Saturday Night Special bans, and all sorts of other laws, but a wholesale ban on all firearms would result in huge numbers of legislators being kicked out. Think we've never had a democrat majority before, with a democratic president? Clinton had a democrat majority for his first two years, and we all know he isn't any sort of lover of guns. Jimmy Carter had a Democrat majority during his entire term. JFK did, too. This isn't anything new, and it's definitely not a once in a lifetime event. We will have anti gun legislation and rulemaking if Obama wins. But we will not have a total ban on all guns in January. I'm not saying a total ban could never happen; just that it would take a lot more than the whims of a newly-elected president.