US Senate Democrats on Wednesday unveiled a historic plan to extend health coverage to more than 30 million Americans who lack it now and set the stage for a key test vote as early as this weekend.

US President Barack Obama, who has made such an overhaul his top domestic priority, hailed the new legislation as "a critical milestone" that brought the United States "closer than ever" to a better health care system.

"The challenges facing our health care system aren't new - but if we fail to act they'll surely get even worse," said Obama, who had sent Vice President Joe Biden and other top aides to lobby wavering senators throughout the day.

The non-partisan Congressional Budget Office says the plan will cost 849 billion dollars over ten years, while extending coverage to 31 million Americans who currently lack health coverage, said a Democratic aide.

CBO's preliminary estimate was that it would also cut the federal budget deficit by 127 billion dollars over the next decade, and 650 billion the 10 years after that, the aide said on condition of anonymity.

Democrats on paper have the 60 votes needed to win on a procedural vote, as early as Saturday, to formally launch the debate but must keep two independents and a handful of waverers in their own party to do so.

"We have traveled a long ways to where we are, and tonight begins the last leg of this journey," Democratic Senate Majority Leader Harry Reid said with guarded optimism. "The finish line is really in sight."

"Absolutely," Democratic Senator Tom Harkin, who became chairman of the Senate Health, Labor and Pensions Committee after Democratic icon Ted Kennedy died in August, said when asked whether Democrats would prevail this weekend.

Republicans seemed united in opposition to the measure, with Senate Minority Leader Mitch McConnell condemning it as "another trillion-dollar experiment" and warning "the American people know that is not reform."

The House of Representatives approved its own trillion-dollar version of the legislation on November 7, squeaking through on a 220-215 margin only after toughening restrictions on federal funds subsidising abortions.

The Senate version allows a government-backed "public option" health insurance plan to provide abortion but bars federal monies from paying for the procedure, a Democratic aide said.

If, as expected, the House and Senate approve different versions, they would need to work out their differences and approve the same legislation to send to Obama to sign into law.

"I look forward to working with the Senate and House to get a finished bill to my desk as soon as possible," said the US president.

"From day one, our goal has been to enact legislation that offers stability and security to those who have insurance and affordable coverage to those who don't, and that lowers costs for families, businesses and governments across the country," he said, stressing the bill "meets those principles."

The United States is the world's richest nation but the only industrialized democracy that does not ensure that all of its citizens have health care coverage, with an estimated 36 million Americans uninsured.

And Washington spends vastly more on health care - both per person and as a share of national income as measured by Gross Domestic Product - than other industrialized democracies, according to the Organization for Economic Cooperation and Development.

The United States spent about 7,290 dollars per person in 2007, more than double what Britain, France and Germany spent, with no meaningful edge in the quality of care, and lags behind OECD averages in key indicators like life expectancy and infant mortality.