Jonathan Gruber is the Ford professor of economics at the Massachusetts Institute of Technology, director of the Health Care Program at the National Bureau of Economic Research and president of the American Society of Health Economists. He consulted with the Obama administration and Congress on the drafting of the Affordable Care Act. Angela Kilby is an assistant professor of economics at Northeastern University and a postdoctoral fellow in aging and health economics at the National Bureau of Economic Research.

Over the past two decades, the number of Americans dying each year from opioid overdoses has quadrupled. In the hardest-hit state, West Virginia, where the overdose death rate is about three times the national average, the crisis has resulted in an overwhelmed foster-care system and a state burial program for the poor that ran through its entire annual budget three months into the year.

This epidemic is one of the most critical public-health issues facing the United States today, but Republicans in Congress seem uninterested in truly grappling with it. Under their proposals to replace the Affordable Care Act, a situation that had been slowly improving would get much worse.

Before the ACA was implemented in 2014, most impoverished adults with substance abuse issues — no matter how poor — had limited access to health coverage because Medicaid doesn’t treat substance abuse as a qualifying disability for benefits. Opioid overdose death rates rapidly escalated, but treatment rates remained flat.

Once the ACA was fully implemented, states that expanded Medicaid were able to achieve significant strides in reducing the destruction of opioid addiction. This was in part thanks to cost-effective and evidence-based addiction treatment, which combines counseling and behavioral therapy with medications such as methadone and buprenorphine. Dozens of clinical trials have shown that such an approach improves outcomes for people struggling with addiction — by up to 34 percent, according to some accounts.

But the same isn’t true for states that didn’t expand Medicaid. Take Ohio and Utah: Before the Medicaid expansion, both faced comparable burdens of opioid addiction and employed similar levels of medication-assisted treatments. But Ohio, which expanded Medicaid, was able to increase its treatment use by 26 percent, while treatment in Utah, which did not expand Medicaid, declined by 6 percent.

The good news is that, primarily due to changes in Medicaid, the United States has more than doubled its total spending on evidence-based substance-abuse treatment for the poor. The bad news is that even with that success, we are facing a massive shortfall in addressing the needs of those with opioid addiction issues. Four out of five people with opioid use disorder do not receive treatment, which means there are nearly 2 million Americans who are addicted to opioids and do not benefit from the proven treatment that could help them.

We need a comprehensive strategy to build on the insurance expansions of the ACA to reach those millions of Americans. The original Senate health-care bill, however, proposes a rollback of the Medicaid expansions that had finally slowed the rapid growth of this devastating problem.

This has gotten the attention of Republicans from hard-hit states, such as Sen. Shelley Moore Capito of West Virginia, who criticized the bill because it “does not do enough to combat the opioid epidemic that is devastating my state.” Capito’s concern is well-founded: The expansion in Medicaid was associated with a 16 percent rise in use of medication-assisted treatment in West Virginia.

Senate leadership is apparently hoping to address such concerns by boosting the funding for treating opioid abuse to $45 billion over the next decade. But this fig leaf falls massively short of addressing the extent of this crisis, which experts estimate would require more than four times as much money.

Furthermore, this fund would be at the political whim of the congressional appropriators. We have seen this show before: The ACA included a large public-health fund of $15 billion to complement insurance expansions. But these funds have been raided over time, with half of the funds diverted to other purposes. Indeed, the Senate and House bills would eliminate the money altogether. What is to stop Congress from doing the same to the stream of opioid treatment funding in the future?

Since the passage of the ACA, we have made promising progress, but these gains also illustrate the magnitude of work needed to address our dire 80 percent treatment gap. The next step is remarkably straightforward: Save lives immediately by bringing the non-Medicaid expansion holdouts into the fold so that they can easily and efficiently provide substance-abuse treatment to our at-risk populations. Instead, the Senate proposes we move backward.

Lost opportunities for substance-abuse care in non-Medicaid expansion states have already cost countless lives. Repealing the ACA would take away access to treatment from hundreds of thousands more. This would be nothing less than a grave moral failing and a travesty for our country.