Start with Obama voters.
The thing that is common among the Tucson Shooting, the Aurora shootings and Sandyhook is not that they used guns, it is that they all involved seriously mentally ill young men who were simply not getting the proper level of care. For centuries, mental illness was viewed a a character flaw, or the result of bad parenting, etc. It's only in the last 30 years of so that we are understanding that these illnesses are real, biological illness, like diabetes. The difference being, these biological illnesses affect the organ we call the brain, which in tun affects complex behaviors.
It is time to treat mental illness with the same level of care and compassion that we treat other serious illnesses.