As a matter of fact that is exactly what the Republicans have done. They reversed the Supreme Court by packing it with conservatives.
I don't think asking that a Democratic president commit to fixing that, rather than acquiesce to it, is unreasonable.
He has to become President first.
Yes, "I'll pack the court with judges who will reverse this decision!" is a winning campaign pledge.
There are a ton of Democrats out there who agree with the decision. Unless you're Carolyn McCarthy, no one's going to be campaigning on gun control these days. For all the b.s. talk about "elitism," it's time others realize that not everyone sees it their way on guns.