But as the C.B.O. report highlighted, the expansive approach Mr. Sanders and Ms. Jayapal have embraced is not the only way to devise a single-payer system. Congress could opt to provide all Americans with coverage more similar to what people 65 and older currently receive under Medicare, with more limited benefits and a requirement that they pay some deductibles and co-payments. A single-payer system could preserve some role for private insurance, either to cover certain benefits or to pay for private care outside the standard system. Such decisions could have a big effect on the overall cost.

When it came to particulars of those costs, however, the budget office said little. “Government spending on health care would increase substantially,” the paper noted at one point. But it never said by how much. The amount matters because it will influence how much tax revenue will be needed to pay for the program. Supporters of a single-payer plan note that, even though government spending would increase, there could be substantial reductions in the other ways individuals and employers pay for health care now through premiums, out-of-pocket spending and state taxes.

The budget office may still provide firm estimates for a proposal if one gets closer to a floor vote in the House or the Senate. The office is charged with developing estimates for legislation, and it produces them even when doing so involves a fair bit of speculation. In past years, for example, the budget office was asked to provide cost estimates for a federal terrorism reinsurance program, which required it to gauge the likelihood of terrorist attacks and the possible expense of their damages.

The cost of a single-payer system is not as unpredictable as that of terrorism insurance, but the report’s many caveats and questions highlight how the effects of Medicare for all will depend on a multitude of legislative decisions — and then a larger set of management decisions by the government that runs the system.

Would government insurance cause shortages of doctors or waits for care? It depends on how well the system pays clinicians, how individuals respond to more generous health coverage, and how the Medicare system adapts over time.

“If the number of providers was not sufficient to meet demand, patients might face increase wait times,” the report noted. But it said such problems were not inevitable under a government-run system: “In the longer run, the government could implement policies to increase the supply of providers.”

Would the government eliminate the denials and other red tape that annoy Americans about the private health insurance system? Maybe, or maybe not. The paper notes that requiring patients to see a primary care doctor before a specialist; denying a treatment that is unusual; or requiring patients to try less expensive drugs before more expensive alternatives would all be possible under single-payer, and are limitations with such systems in other countries.