In May 1965, just weeks before Lyndon Johnson signed Medicaid into law, his administration launched Head Start, an enrichment program for preschool-aged children from poor families.

The program’s administrators were appalled by the poor health of their students. In Jacksonville, Florida, more than half of participating children were anemic, and between one-quarter and one-third suffered hearing and sight problems. In Beaufort County, South Carolina, 90 percent of kids suffered from hookworms and roundworms. In Boston, almost one-third of Head Start youngsters showed signs of physical or mental health illnesses.

Rotting teeth, vitamin deficiency, chronic infections—50 years ago in the United States, this was how many 3- and 4-year-olds from poor families lived.

Then came Medicaid—an afterthought tacked onto the administration’s Medicare bill, and one that LBJ scarcely mentioned when he signed both measures into law. Medicaid’s roots were humble, its ambitions modest. As originally conceived, the program provided health insurance to poor children, poor pregnant women and some qualifying parents. In its first year, its budget was less than $1 billion—about $7.7 billion in today’s dollars.

Over 50 years, successive Congresses and presidential administrations vastly expanded the program’s scope to cover 80 million people, or almost one-quarter of the population. Its budget last year was $378 billion. To be sure, it has never enjoyed the popularity of Medicare, which covers a much more politically powerful constituency: seniors. But it has proven highly durable. Despite the GOP’s preference for smaller government and lower taxes, for many decades, Medicaid enjoyed broad backing from Republican leaders.

That was then. Now, congressional Republicans have proposed taking a hacksaw to Medicaid—a move that will, according to the Congressional Budget Office, leave many millions of poor people uninsured. For Democrats, the Senate GOP’s Better Care Reconciliation Act,which ends Medicaid’s entitlement status, is a “monstrosity.” For many Republicans, it is nirvana. “You and I have been dreaming of this since I have been around, since you and I were drinking at a keg,” House Speaker Paul Ryan told Rich Lowry, editor of National Review, earlier this year. That position isn’t widely popular: Amid much public outcry, the Senate postponed its anticipated vote on BCRA this week. But it testifies to the GOP’s full metamorphosis from the Party of Ronald Reagan to the Party of Ayn Rand.

Still, there’s more to the story. Republicans are right to observe that Medicaid was never supposed to grow as big as it did. Its framers intended the program to help a small few who were unable to capture the full benefits of America’s postwar prosperity. In their hubris, 1960s liberals assumed that a booming economy would continue to grow in perpetuity. They didn’t anticipate industrial decline, growing inequality, an explosion of single-parent households or the contraction of America’s unionized workforce and, with it, employer-based health insurance. ...