A sweeping mental health overhaul cast as a congressional response to gun violence is running afoul of … gun control.

Which is precisely what happened the most recent time lawmakers tried to pass mental health legislation, after the 2012 Sandy Hook school massacre in Newtown, Connecticut.

The spate of mass killings over the past year reignited mental health reform efforts in both chambers of Congress. A bipartisan bill is gaining momentum in the Senate, with the Committee on Health, Education, Labor and Pensions likely to take it up early next year. The House Energy and Commerce subcommittee on health recently approved a similar bill, and Speaker Paul Ryan this month said on “60 Minutes” that he wants Congress to move ahead on mental health.

But the Senate’s No. 2 Republican, John Cornyn of Texas, has been working behind the scenes to drum up support for his own mental health legislation, which includes language endorsed by the National Rifle Association.

Cornyn says his bill would boost the federal background check system to prevent guns from getting into the hands of those with serious mental illness. His critics say the legislation actually loosens restrictions on gun purchases, under the umbrella of mental health reform.

“The net effect of this bill would be to weaken, not strengthen, our background check system, and make it easier for people struggling with dangerous mental illness to legally access a gun,” said Mark Prentice, spokesman for Americans for Responsible Solutions, former Rep. Gabby Giffords’ gun control advocacy group.

Cornyn told POLITICO he expects a hearing in the Judiciary Committee in January — and that he believes his bill will become “the engine that pulls the train” on mental health.

Any push to include guns could create a wedge in the bipartisan coalition that has been working on a mental health overhaul that doesn’t involve the politically volatile issue.

Cornyn confirmed he’s talked to Sen. Bill Cassidy (R-La.), a sponsor of the main bipartisan mental health initiative, about packaging the two bills together. A Cassidy spokesman said Cassidy is “supportive” of that idea. HELP Committee Chairman Lamar Alexander (R-Tenn.) has vowed to hold a markup on the Cassidy mental health legislation, but he also recently floated the possibility of blending several bills into one big package.

But Sen. Chris Murphy (D-Conn.), co-sponsor with Cassidy of the bill gaining traction in HELP and a progressive who is pushing separate gun control legislation, is not on board with incorporating Cornyn’s measures.

Murphy “thinks there are a lot of good ideas on the table,” his spokesman Chris Harris said. «But ultimately, he believes the final product has the greatest chance of passage if it maintains its singular focus on mental health, rather than venturing into other complex issues.”

The possibility of a schism is disturbing both to mental health advocacy groups — many of whom didn’t want to be quoted criticizing Cassidy or Alexander as they move the issue ahead — and to gun control advocates. The last time the Senate tried to pass ambitious mental health legislation, after Newtown, the bipartisan effort ground to a halt.

Sweeping proposals were whittled down, and what was left failed when it was tacked onto a gun background check measure that was defeated on the floor in 2013.

Since then, neither the full Senate nor the House has considered comprehensive mental health legislation, although a few more modest initiatives have passed.

Neither the Cassidy-Murphy bill nor the House version — sponsored by Rep. Tim Murphy (R-Pa.) — is a done deal; there are plenty of disputes to resolve over patient privacy, court-ordered outpatient treatment and the price tag for an overhaul. But both pieces of legislation had begun to move as lawmakers looked for a way to respond to the mass shootings across the country.

Sen. Debbie Stabenow (D-Mich.), who has long advocated both mental health legislation and tougher gun laws, wants to keep the two initiatives separate. On mental health, she sees a chance at finding the common ground that’s so elusive in Washington. But not if guns are added to the mix.

“This is something that needs to be done and stand on its own,” Stabenow said.

Gun control activists say Cornyn’s language allows for patients’ gun rights to be restored immediately upon their release from involuntary treatment. In addition, they say it narrows the definition of who would actually be prohibited from buying a gun.

Cornyn’s measure expands upon a grant program that gives incentives to states that share mental health records with the National Instant Criminal Background Check System, known as NICS. The goal is to prevent anyone who has been adjudicated by a court as mentally ill from buying a gun.

On its website, the NRA says Cornyn’s bill would “protect the Second Amendment rights of law-abiding citizens from continued bureaucratic abuse by the Obama Administration.” Under the system laid out in the bill, prohibiting someone from possessing a gun requires a “full hearing in which an individual has notice, the opportunity to participate, and the right to counsel.”

Cornyn’s bill includes other mental health provisions, and he incorporated elements of Sen. Al Franken’s bill on mental health and criminal justice. But the Minnesota Democrat won’t back Cornyn’s legislation because of the gun element.