Mass shootings.
If mass shootings stopped or became much more rare, the antigun movement would wither away. They are totally reliant on those attacks to try and bottle some mass hysteria to pass draconian gun laws, and it's the only time the general public (temporarily) pays attention to orgs like Everytown or Giffords.
Most people intuitively understand that "routine" gun-related crime is, aside from tragic incidents of people being caught in crossfire, avoidable. If you aren't involved in gangs or drugs, and don't go to stupid places at stupid times, then generally, gun-related crime won't affect you, and you are as safe here as you are in any other first world country.
Similarly, people know they can avoid tragedies like a child getting ahold of a gun and shooting himself or another by responsibly storing firearms.
But mass shootings can reach you anywhere even if you are doing nothing irresponsible or criminal, and that understandably scares people. It doesn't really matter that such shootings are statistically super rare, and some years lightning kills more than mass shootings do. In the same way terrorist attacks have an outsize effect on public policy relative to their bodycount, mass shootings are the gun ban movement's gasoline.
This is why they push very hard to make the public believe mass shootings are much more common than they are through nonsense like the Gun Violence Archive's expansive definition (their data mostly reflects large gang-related shootings).
It's also why they are so very angry when an armed citizen stops or cuts short a mass shooting - as that citizen just cost them their main PR opportunity! No surprise then that they do everything possible to make sure an armed citizen isn't there to stop shootings, by supporting laws that make getting carry permits more difficult, and laws that ban carry in as many places as possible.
We should try to stop mass shootings because it's the right thing to do, nobody should ever be killed because of some deranged loser nutjob trying to murder random innocents. But in stopping or reducing them, we'd also defeat the main engine of authoritarian gun control.
What do you think is the biggest threat to the 2nd Amendment today?