Not so long ago, the scourge of opioids seemed unstoppable. More than 400,000 Americans died from drug overdoses between 2020 and 2023. The toll was more than twice as large as that from either guns or vehicle accidents. But 2023 now appears to have been a turning point. Since then, annual overdose deaths have declined more than 25 percent, thanks partly to a creative public health campaign to expand access to treatments like Narcan and Suboxone. The crisis is finally easing.

President Trump’s big domestic policy law threatens that progress. The law’s Medicaid cuts, which finance lower taxes for the wealthy, will deprive millions of Americans of health insurance. These changes will harm people with all sorts of medical conditions. Yet addicts are particularly vulnerable because of how many of them are on Medicaid. The program covers nearly half of non-elderly adults with an opioid addiction, according to KFF, a health research group. Without insurance, many will drop out of treatment and relapse. Researchers at Boston University and the University of Pennsylvania estimate that the law will end access to opioid treatment for more than 150,000 Americans.

The recent fall in overdose deaths should be a cause for celebration, and one that the country’s leaders should look to build on. Opioids have been an important factor in the shocking stagnation of American life expectancy in recent decades. In the 1980s, life expectancy here was similar to the levels in many other rich countries; today, the United States comes in last. No other wealthy nation experienced a similar spike in overdoses in the 2000s.

Mr. Trump, Vice President JD Vance and other Republican leaders have rightly described opioids as a national tragedy that demands action. Instead of taking steps to continue the recent progress, however, they are undermining it. This is one more way in which they are failing to live up to their promise to govern as champions of the working class that voted for them in large numbers last year.

This is not the first time that America’s political leaders have failed to take the opioid crisis seriously. After Purdue Pharma, a company owned by the Sackler family, introduced OxyContin in 1996, the painkiller quickly became the drug at the center of the epidemic. Overdose deaths doubled between 1999 and 2006. Still, Congress did not pass major legislation to address the crisis until 2016. Not until 2017 did a president, Mr. Trump, declare a national emergency. Even these actions produced underwhelming results — what Dr. Leana Wen, a former health commissioner of Baltimore, once described as “tinkering around the edges.”