But experts say dividing the insurance risk pool in this way would force insurers to raise premiums a lot, because plans that cover more services would primarily attract people who have more health problems. Many middle-class families would not be able to afford those plans, since they earn too much to qualify for subsidies. This so-called compromise smacks of the kinds of changes demanded by the Freedom Caucus, whose members objected to Speaker Paul Ryan’s original bill in the House. As a result of their demands, the measure passed by the House would let insurers discriminate against people with pre-existing conditions.

In reality, the lawmakers who will make or break the Senate bill are centrist Republicans and lawmakers who represent states that expanded Medicaid under the A.C.A. These groups include Susan Collins of Maine, Shelley Moore Capito of West Virginia, Cory Gardner of Colorado, Dean Heller of Nevada, Lisa Murkowski of Alaska and Rob Portman of Ohio. Mr. McConnell, President Trump and other Republicans are putting lots of pressure on them — and are trying to win them over with modifications that may seem like improvements but do not change the bill’s substance.

For example, Mr. McConnell has reportedly agreed to allocate $45 billion over 10 years to deal with the opioid epidemic, up from $2 billion in his original proposal. This change is aimed at winning the support of Republican senators from states that are struggling with the scourge of addiction. Granted, this would be a big increase, but experts say it is hardly enough given the scope of the problem. And it cannot make up for the faults in the rest of the bill, which would most hurt people in states at the epicenter of the opioid crisis.

The Congressional Budget Office estimates that the legislation will take health insurance away from 22 million people by 2026, including 15 million who will be kicked off Medicaid. More federal spending on the opioid problem might help some of those people get addiction treatment if they need it, but it will not help them if they happen to need, say, chemotherapy, insulin or heart surgery.