The expansion of Medicaid, a central pillar of the Affordable Care Act, faces immense uncertainty next year, with President-elect Donald J. Trump and top Republicans in Congress embracing proposals that could leave millions of poorer Americans without health insurance and jeopardize a major element of President Obama’s legacy.

But influential figures in surprising quarters of the new administration might balk at a broad rollback of Medicaid’s reach, favoring new conditions for access to the government insurance program for the poor but not wholesale cutbacks.

Mike Pence, the vice president-elect, is proud of the Medicaid expansion he engineered as governor of Indiana, one of 31 states that expanded eligibility under the Affordable Care Act. The Indiana program has conservative features that emphasize “personal responsibility” and require Medicaid beneficiaries to make monthly contributions to savings accounts earmarked for health care.

Another Trump adviser, Gov. Chris Christie of New Jersey, hails the expansion of Medicaid in his state, saying more than half a million people are receiving “more and better health care.” The federal government pays the full cost for newly eligible beneficiaries from 2014 through 2016 and at least 90 percent of the costs in later years under the health law.

It is hard to overstate the importance of Medicaid, which insures 77 million people, pays for more than half of all births in some states, covers about two-thirds of nursing home residents and provides treatment for many people addicted to opioids. Spending on Medicaid, by the federal government and states combined, exceeds $500 billion a year.

Of the 20 million people who have gained coverage under the Affordable Care Act, officials estimate, 12 million are insured by Medicaid — with few of the problems that have plagued the new insurance exchanges, or marketplaces.

But change is coming. In his campaign manifesto, Mr. Trump said Congress must repeal the Affordable Care Act and give each state a lump sum of federal money — a block grant — for Medicaid. Congress passed legislation in January to repeal the health law and roll back its Medicaid expansion. Mr. Obama vetoed the measure, but Speaker Paul D. Ryan of Wisconsin has vowed to put similar legislation on a Republican president’s desk.