Trump voters will be especially hard hit if just part of this sweeping agenda comes to fruition.

Conservatives often point to poor blacks and Latinos as the primary beneficiaries of federal health insurance programs. But such rhetoric obscures the enormous importance of Medicaid, Medicare and Obamacare subsidies to economically struggling white Americans living in small cities and rural areas. In Pennsylvania, where Mr. Trump narrowly beat Hillary Clinton with overwhelming support outside big cities, about 17 percent of residents are 65 or older, above the national average. Meanwhile, some 16 percent of Pennsylvanians benefit from Medicare, and 18 percent from Medicaid. With the bulk of Medicaid going to elderly and disabled residents, that program is the single largest federal subsidy flowing into the Keystone State.

Repealing the Affordable Care Act would also hit Pennsylvania hard. Under the act, some 468,000 low-income Pennsylvanians had gained Medicaid coverage by August 2016, and another 439,000 bought private coverage on the Obamacare marketplace, with more than three-fourths of those people getting tax credits averaging $251 per month. Health care is often sparse in nonurban areas, and the providers that do exist depend on federal insurance programs that help many patients pay for care. If radical Republican cutbacks in federal contributions to health insurance are enacted, Pennsylvania hospitals and health care businesses will lose vital revenues, leaving many lower-income and sick Pennsylvanians at risk of losing access to care.

This is the case in other states as well, meaning many rural and small-town Trump supporters may soon see that Make America Great Again means accelerating economic decline and social devastation. Mr. Trump shows little understanding of the intricate interplay of subsidies and rules in the health care system, and probably has no inkling that federal taxes collected from liberal states like California, Massachusetts and New York heavily subsidize vital health services, businesses and family benefits in the very places that voted heavily for him. In delegating plans for huge health care cutbacks to hard-right congressional Republicans, he will be hurting his own base.

But will Mr. Trump suffer repercussions if the Republican Congress plows ahead? Its proposed changes are unpopular — including repealing the Affordable Care Act, which only one in four Americans support — and eliminating benefits usually arouses anger in the affected groups. But political punishment will not be automatic, because Democrats currently have little organized presence outside urban areas. Small cities and rural areas are overwhelmingly represented in Congress and state capitols by Republicans, who will do all they can to displace blame.

For the Democratic Party, the coming Republican assault on public health insurance represents a huge political opportunity. But to seize it, the party will have to beef up state committees and place a priority on activating volunteer supporters everywhere — getting people to write messages to local newspapers and social media sites, and reach out to hospitals, health care providers and nonprofits to beat the drums about losses the Republicans are inflicting. Even if Democrats cannot soon win outright majorities beyond their urban base, they must be actively involved in communities damaged by Mr. Trump’s false campaign promises.