Harold Pollack is a professor at the School of Social Service Administration at the University of Chicago. This is a guest column.

Oregon’s mass homicide sparked the usual debate about whether guns or mental health is the best focus in preventing atrocities. Given this stark frame, the centrality of gun policy is hard to deny. Compared with other wealthy democracies, America has surprisingly similar rates of car theft, aggravated assault, and other forms of nonlethal violence. Our gun homicide rate is about three times the average among our peers. Gun policy measures such as improved background checks included in the nearly-passed, bipartisan, post-Newtown Manchin-Toomey bill would certainly be helpful.

Many conservatives place greater emphasis on the mental health system. In some ways, this rhetoric is misplaced. The fraction of American violence attributable to severe mental illness is quite low, on the order of five percent. We must also avoid reinforcing cruel stereotypes regarding millions of Americans who live with major depression, bipolar disorders, or related conditions.

Still, it’s always wise to consider how our mental health systems could treat people more effectively, and more-reliably keep weapons away from dangerous individuals. I’ve discussed some options here at Wonkblog in the aftermath of the Isla Vista shootings. One useful initiative would be to expand the power of police and mental health professionals to temporarily confiscate guns from individuals whose behavior raises real concerns, but who do not meet the stringent criteria required to justify involuntary commitment or other coercive interventions.

Texas Senator John Cornyn has proposed legislation, the mental Health and Safe Communities Act, that would, among other things, expand states’ provision of mental health information to the National Instant Criminal Background Check System. Responding to President Obama 13-minute statement on gun violence on Thursday, Senator Cornyn tweeted,

The potential impact of such data-sharing remains unclear. Only a small minority of dangerous individuals have documented histories of involuntary commitment or severe mental health crises. Cornyn’s bill also does not address private transactions in which no background check is performed.

Some aspects of Cornyn’s proposal are helpful and constructive. It would support new screening and treatment, and better crisis response strategies. Our background check system could certainly benefit from improved management and financial resources.

Cornyn’s proposal does not address the most glaring issue in American mental health policy: the Affordable Care Act’s Medicaid expansion. Medicaid expansion was always the public health cornerstone of ACA. It remains the single most important measure to expand access to mental health and addiction treatment, serving severely vulnerable populations such as the homeless, addressing the complicated medical and psychiatric difficulties of many young men cycling through our jails and prisons.

Here in Chicago, Sheriff Tom Dart has played a leading role in efforts to serve thousands of people with psychiatric disorders who pass through the Cook County Jail. Three years ago, almost all of these detainees were uninsured. Now, because of Medicaid expansion, the great majority gain health coverage before they are discharged.

Because they are now insured, these men and women have greater access to medical, psychiatric, and addiction services. Hard work is now underway to field interventions that make the most of this new access. Ohio Governor John Kasich eloquently describes the importance of such efforts.

More subtly, Medicaid expansion provides financial stability to the whole network of safety-net medical, psychiatric, and addiction care. The Chicago Tribune reports that the Cook County Health System is experiencing its first surplus in decades. For the first time in memory, the majority of low-income patients are insured. This story extends far beyond Chicago, too. Hospital losses from uncompensated care have sharply declined across the Medicaid expansion states.

Safety-net providers and patients experience greater difficulty in non-expansion states. About four million low-income adults are uninsured because their state has declined to expand Medicaid, even though the federal government would pay more than 90 percent of the bill. Twenty-six percent of adults caught in this “Medicaid gap,” almost one million people, live in Texas, Senator Cornyn’s own state.

In 2013, the National Alliance on Mental Illness (NAMI) released a report endorsing Medicaid expansion. The report argued that “States that decline to expand Medicaid will miss as good an opportunity as they may ever have to address this shameful void in access to mental health treatment.” Addressing the connection between mental illness and violence, NAMI concluded:

In the aftermath of Newtown, many politicians and policy makers have promised to take steps to fix America’s broken mental health system. Expanding Medicaid in all states would represent a significant step towards keeping those promises.

Senator Cornyn is an implacable opponent of Medicaid expansion. Indeed he rallied at the Texas state capitol to oppose it.

No one knows the precise prevalence of addiction and psychiatric disorders within the population of Texans deliberately left uninsured. Extrapolation from national data suggest that roughly 140,000 live with addiction disorders, and that roughly 54,000 live with severe mental illness. This population includes indigent criminal offenders and those seeking care at addiction treatment centers and stressed safety-net facilities that have lost billions of dollars because Texas has declined Medicaid. Leaving aside the human consequences for the uninsured, this is very poor violence prevention policy.

If any other politician suggests that mental health rather than gun policy is central to reducing mass homicides, ask where they stand on Medicaid expansion. Their answer will be clarifying.