WASHINGTON — If President Trump allows states to convert Medicaid into a block grant with a limit on health care spending for low-income people, he will face a firestorm of opposition in Congress, House Democrats told the nation’s top health official on Tuesday.

The official, Alex M. Azar II, the secretary of health and human services, endured more than four hours of bipartisan criticism over the president’s budget for 2020, which would substantially reduce projected spending on Medicaid, Medicare and biomedical research. Democrats, confronting Mr. Azar for the first time with a House majority, scorned most of the president’s proposals.

But few drew as much heat as Mr. Trump’s proposed overhaul of Medicaid. His budget envisions replacing the current open-ended federal commitment to the program with a lump sum of federal money for each state in the form of a block grant, a measure that would essentially cap payments and would not keep pace with rising health care costs.

Congress rejected a similar Republican plan in 2017, but in his testimony on Tuesday before the Health Subcommittee of the House Energy and Commerce Committee, Mr. Azar refused to rule out the possibility that he could grant waivers to states that wanted to move in that direction.