Now that we've become so utterly desensitized to these types of events, the Democrats should just drop the whole gun control issue. It's over anyway. When the (R)'s stole that SCOTUS seat, the notion that anything sensible would be done regarding guns ended in all of our lifetimes.
On liberal gun forums (yes, they do exist), you'll find lots of leftists who are just as one-issue as the stereotypical NRA loving, drooling redneck who wants to open carry his tricked-out AR with him wherever he goes. Here's an example to peruse at your leisure:
https://www.reddit.com/r/liberalgun.../two_dead_17_injured_in_kentucky_high_school/
Note that the discussion is much more reasoned and varied than you'll find on a standard gun forum (e.g. cal guns.net), but the majority opinion is the same: bad people will find ways to do bad things, don't take away my rights because someone else did something bad, etc.
Gun control hasn't won the Democratic party any votes. Instead, it's cost them. And the thing is, they can't do anything to change it. There's 300,000,000 or so guns in the hands of private ownership in the U.S. There's not going to be any mass outlawing of them, there isn't enough money to buy them back, there's no criminal laws tough enough on the books to deter their illegal use, etc.
So give it up. Drop it as a plank completely. This is one that has been left squarely in the hands of the people and the people have decided that no matter what, gun control like we see in Europe and Japan is not going to happen here. There is no act too horrific to change enough enough minds.
Put it in the hands of the individual states. Sure, that's relatively futile in terms of getting gun murders under control given the makeup of SCOTUS because the 2nd Amendment as well as issues relating to interstate commerce fall under federal law, but this is a lost fight anyway.
Sandy Hook happened and not a single meaningful reform came from that. That, coupled with Gorsuch and Trump's election settled the matter for good.