Mr. Trump and Republicans in Congress failed this year in their efforts to cut Medicaid and could try again in 2018. Speaker Paul D. Ryan said this month that Republicans would try next year to slow the growth of federal health spending because “it’s the health care entitlements that are the big drivers of our debt.”

But Senator Mitch McConnell, the majority leader, has been leery of another run at health care in an election year, and it is possible that Medicaid could, in the near future, actually grow further under the Affordable Care Act. Maine voters approved a referendum last month to expand Medicaid in that state, though Maine’s Republican governor, Paul R. LePage, is dragging his feet. Mr. LePage will not be on the ballot in November, when the state could elect a governor more willing to accept the referendum’s results.

The governor-elect of Virginia, Ralph S. Northam, has vowed to expand Medicaid and could have an evenly split House of Delegates, depending on the outcome of a drawing of lots this week that will decide the winner of a tied race in southeastern Virginia.

A Democratic wave election in November could also shift the balance of power in a few other state capitals.

While the marketplaces, or exchanges, have struggled with a series of problems since they opened in 2014, Medicaid, administered by an experienced corps of state officials, has gone from strength to strength. Public appreciation for the program has steadily increased as people come to understand its importance in the health care system, including its central role in combating the opioid epidemic.

And though Congress has effectively repealed the requirement for people to have health insurance, federal subsidies are still available to low- and moderate-income people who want insurance. The federal government pays, on average, about three-fourths of the premium for more than three-fourths of the people who buy insurance through the Affordable Care Act marketplace.