When it was created more than a half century ago, Medicaid almost escaped notice.

Front-page stories hailed the bigger, more controversial part of the law that President Lyndon B. Johnson signed that July day in 1965 — health insurance for elderly people, or Medicare, which the American Medical Association had bitterly denounced as socialized medicine. The New York Times did not even mention Medicaid, conceived as a small program to cover poor people’s medical bills.

But over the past five decades, Medicaid has surpassed Medicare in the number of Americans it covers. It has grown gradually into a behemoth that provides for the medical needs of one in five Americans — 74 million people — starting for many in the womb, and for others, ending only when they go to their graves.

Medicaid, so central to the country’s health care system, also played a major, though far less appreciated, role in last week’s collapse of the Republican drive to repeal and replace the Affordable Care Act, also known as Obamacare. While President Trump and others largely blamed the conservative Freedom Caucus for that failure, the objections of moderate Republicans to the deep cuts in Medicaid also helped doom the Republican bill.

“I was not willing to gamble with the care of my constituents with this huge unknown,” said Representative Frank A. LoBiondo of New Jersey, a member of the centrist Tuesday Group caucus, noting that in three of the counties in his district in the state’s more conservative southern half, over 30 percent of all residents are covered by Medicaid.