As Senate Republicans resume their drive this week to repeal former President Barack Obama’s Affordable Care Act, one of their principal obstacles is resistance from GOP senators and governors in states that expanded Medicaid under the law. And one of the principal reasons for that opposition is Medicaid’s central role in responding to the opioid challenge. Federal data show that Medicaid now pays for about one-fourth of all substance-abuse treatment, up from about one-tenth in 1986. “Losing expanded Medicaid coverage would absolutely hamstring our operations to address opioid addiction in eastern Kentucky,” Green said. “We are critically dependent on that.”

New data compiled by the Center on Budget and Policy Priorities, and provided exclusively to The Atlantic, underscores just how important Medicaid is for many of the places struggling most with opioids.

The CBPP, a liberal research and advocacy group, analyzed Centers for Disease Control and Prevention data on overdose deaths per capita in the counties of four heartland states confronting the opioid crisis: Kentucky, West Virginia, Ohio, and Arkansas. Then it used Census data to measure what share of each county’s population receives health care through Medicaid. The results were striking.

Looking at the 10 counties in Ohio with the highest rates of overdose deaths, Medicaid provides coverage for one-fourth or more of the population in five of them, and about one-fifth in the remaining five. In Arkansas, Medicaid covers about three-tenths of the population in the county with the highest rate of overdose deaths, and between one-fifth and one-fourth in the other nine.

In West Virginia, Medicaid covers more than 30 percent of the population in three of the 10 hardest-hit counties, over one-fourth in five more, and at least one-fifth in the final two. In Kentucky, 17 counties tie for the highest rate of overdose deaths. There, Medicaid covers over two-fifths of the population in five counties and roughly one-third in all the others.

Colleen Grogan, a University of Chicago health-policy professor, notes that Medicaid has become indispensable in these counties for two reasons. First, she said, the ACA extended Medicaid eligibility, for the first time, to low-income working adults: They “never had access to coverage—and many of those people had substance-abuse problems.” Equally important, the ACA required states joining the Medicaid expansion to cover drug treatment.

Like Clay in Kentucky, the counties with the highest overdose rates across the four states are predominantly white and have very few college graduates. Most face economic strain. Last year, Trump carried every one of the highest-overdose counties in each of the four states examined. In fact, Trump won over 60 percent of the vote in all 47 counties except two in Ohio.