The Trump administration immediately denounced the budget office’s conclusions. Tom Price, the secretary of health and human services, suggested the report offered an incomplete picture because it did not take into account regulatory steps he intends to take, as well as other legislation that Republicans plan as part of their multistep strategy to repeal and replace the health law.

“We disagree strenuously with the report that was put out,” he said at the White House.

Democrats remained steadfast in their opposition. “The C.B.O. score shows just how empty the president’s promises, that everyone will be covered and costs will go down, have been,” said Senator Chuck Schumer of New York, the Democratic leader. “This should be a looming stop sign for the Republicans’ repeal effort.”

The coverage numbers released Monday will make it only more difficult for Republicans to explain why their legislation would improve the country’s health care system. And that could make the bill’s fate in the more narrowly divided Senate much more tenuous.

In a sign of the concern over the coverage projections, Senator Susan Collins, Republican of Maine, said the budget office’s report was “cause for alarm” and “should prompt the House to slow down and reconsider certain provisions of the bill.”