President Trump’s plan to replace the Affordable Care Act will propose giving each state a fixed amount of federal money in the form of a block grant to provide health care to low-income people on Medicaid, a top adviser to Mr. Trump said in an interview broadcast on Sunday.

The adviser, Kellyanne Conway, who is Mr. Trump’s White House counselor, said that converting Medicaid to a block grant would ensure that “those who are closest to the people in need will be administering” the program.

A block grant would be a radical change. Since its creation in 1965, Medicaid has been an open-ended entitlement. If more people become eligible because of a recession, or if costs go up because of the use of expensive new medicines, states receive more federal money.

If Congress decides to create block grants for Medicaid, lawmakers will face thorny questions with huge political and financial implications: How much money will each state receive? How will the initial allotments be adjusted — for population changes, for general inflation, for increases in medical prices, for the discovery of new drugs and treatments? Will the federal government require states to cover certain populations and services? Will states receive extra money if they have not expanded Medicaid eligibility under the Affordable Care Act, but decide to do so in the future?