Senator Patty Murray of Washington, the senior Democrat on the health committee, said on Tuesday that she was appalled that the administration refused to defend protections for people with pre-existing conditions, one of the most popular provisions of the Affordable Care Act.

Senator Bob Casey, Democrat of Pennsylvania, said, “I don’t know of any American who wants to go back to those days when you could be denied coverage or treatment because of pre-existing conditions.”

Republicans repeatedly tried and failed to repeal or roll back the health law last year.

The term “pre-existing conditions” refers not just to serious illnesses like cancer. Before the Affordable Care Act, some insurers denied coverage or charged higher premiums to people with high blood pressure, seasonal allergies, diabetes, arthritis and migraine headaches, among other conditions. The Kaiser Family Foundation has estimated that at least one-fourth of Americans below the age of 65 have conditions that could have made them uninsurable under medical underwriting practices used before the Affordable Care Act.

At a bill-signing ceremony on May 30, Mr. Trump said that major drug companies would, within two weeks, announce “voluntary massive drops in prices.”

Mr. Azar told Congress on Tuesday that might not happen on that schedule.

“We had several drug companies come in who want to execute substantial material reductions in their drug prices,” Mr. Azar said. “They are finding hurdles from pharmacy benefit managers and distributors.”

The benefit managers, he said, make money when drug companies set high list prices because the managers receive rebate payments from drugmakers — a percentage of the list price — in return for promoting the use of those companies’ products.

“Everybody wins when list prices rise — except for the patient, whose out-of-pocket cost is typically calculated based on that price,” Mr. Azar said. For this reason, he said, “we may need to move toward a system without rebates, where pharmacy benefit managers and drug companies just negotiate fixed-price contracts.”