Lawmakers can assure health coverage for almost all Americans with either mandatory or voluntary programs, the Congressional Budget Office said as it warned that runaway costs threaten to wreck the government's finances.

The nonpartisan agency laid out the choices in two documents, each around 200 pages, analyzing ways to overhaul health care, a top priority of President-elect Barack Obama. Getting everyone insured and bringing down health costs will require "major" changes in how people get their health coverage, the agency said yesterday.

Whether people should be required to buy health insurance became an issue in Obama's campaign for the Democratic presidential nomination. He said he didn't believe in mandating coverage because people would buy insurance if it was cheap enough. The budget office said a voluntary system could work if everyone received subsidies that covered a large share of their health-insurance costs.

Federal spending on the Medicare health plan for the elderly and disabled and the Medicaid program for the poor makes up about 4 percent of the U.S. economy. This share is expected to rise to about 6 percent in 2019, mainly because of health costs, posing a "serious threat" to U.S. finances, the study said.

Members of Congress and Obama have pledged to rework the $2.2 trillion U.S. health system to lower costs and reduce the 15 percent uninsured rate. Obama this month named the agency's head, Peter Orszag, an expert on health financing, as director of the Office of Management and Budget.

Requiring that all Americans have health coverage won't substantially reduce the number of uninsured unless there are subsidies to help low-income families pay for premiums, according to the report.

Achieving "near-universal coverage" could also be done with a voluntary system combining ease of enrollment with subsidies to cover most insurance costs for everyone, the authors wrote.

The number of people without coverage will rise to about 54 million in 2019, from 45 million now, without any changes in the system, the authors wrote.

Trying to reshape the system to get more people covered at a lower cost to the government, by encouraging people to buy cheaper and less-extensive coverage, could reduce use of some treatments, the study said. It also might leave people stuck with a bigger portion of their medical bill.

This report wasn't based on any specific legislative measure. The office instead evaluated general issues important for reshaping the health system, including how the value of health plans can be computed.

The reports, while they don't cover every option, are critical for policymakers looking into writing legislation that would change how Americans get health coverage, Edwin Park, a senior fellow at the Center on Budget and Policy Priorities.

"It's a menu in some ways of options, and while there are certainly some items not listed, it gives a sense of what the budget office thinks of costs of certain ideas," Park said.