The argument against the mentally ill restriction is that the situation is temporary and hard to define. Depression is often correlated with gun violence (not necessarily against others, of course, but suicides are wrapped into the statistics), yet depression is a fairly vague, nebulous, and often temporary condition. It might be theoretically reasonable to restrict gun possession by depressed people, but it is not practical to implement as a law.

My personal feeling is that we are required to demonstrate a minimal (VERY minimal) competence and obtain a license to operate an automobile due to the potential harm a person could cause with it. We are also required in many states to undergo a far more rigorous training before we are allowed to get a hunting license. I see no good reason not to require the same for gun ownership. The idea that a license would be the first step towards a ban is absurd....until they come for our cars.