WASHINGTON — In the end, Republicans relearned a lesson that has bedeviled them since the New Deal: An American entitlement, once established, can almost never be retracted.

Since the day the Affordable Care Act passed Congress, Republicans have vowed to overturn it. In the beginning, many voters were with them, handing the Republican Party some of the tools: a sweeping rejection of House Democrats in 2010 — a rejection of government reach — followed by the Senate in 2014.

But in the intervening years, as millions of Americans have become insured under the law that was derisively tagged with President Barack Obama’s name, the health care program has become more and more popular, even with Republican governors.

In red states where Mr. Obama and Democrats remain highly unpopular, the law’s reach into American lives could not be denied. This was true for communities ravaged by the opioid crisis, which health care money helped treat; for rural states where hospitals had become all but dependent on increased Medicaid payments that covered the bulk of their patients; and for poor constituents with chronic medical conditions who had come to take it as an article of faith that their insurance companies could not deny them coverage for pre-existing conditions.