WASHINGTON (AP) — A senior Democratic leadership aide says legislation headed to the Senate floor would extend health care coverage to 31 million uninsured Americans at a cost of $849 billion over a decade.

Citing the Congressional Budget Office, the aide says the bill would cut the federal deficit by $127 billion over that decade.

The aide spoke on condition of anonymity because the information had not been shared with rank-and-file Democrats in the Senate.

Debate is expected to begin on the measure this week.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP's earlier story is below.

WASHINGTON (AP) — The political stakes enormous, Senate Majority Leader Harry Reid briefed crucial moderate Democrats before unveiling long-awaited legislation Wednesday to remake the nation's health care system.

Revealing the bill's details signals the beginning of an intense struggle on the Senate floor, where Republicans have vowed to block the legislation atop President Barack Obama's domestic agenda.

Officials have said the legislation would require most Americans to carry health insurance and would mandate large companies to provide coverage to their workers, as well as ban insurance company practices such as denying coverage on the basis of pre-existing medical conditions.

The bill would set up new insurance marketplaces — called exchanges — primarily for those who now have a hard time getting or keeping coverage. Subsidies would be available to help defray the cost of coverage for people with lower incomes.

Reid announced two weeks ago it would also include an option for consumers to purchase government-sold insurance, with states permitted to drop out of the system.

Reid, D-Nev., summoned members of the Democratic rank and file to a late-afternoon closed-door caucus to show the bill he has spent weeks writing and rewriting.

He met in advance with Sens. Ben Nelson of Nebraska, Mary Landrieu of Louisiana and Blanche Lincoln of Arkansas, Democratic moderates who have expressed reservations about the bill. "He is walking through the particulars with them," said Reid's spokesman, Jim Manley.

"We need 60 votes to get this bill to the floor."

With the support of two independents, Democrats have 60 seats, the precise number needed to choke off any Republican delaying tactics. None of the 40 Republicans is expected to defect on the first test vote, expected by weekend.

Ahead lie weeks — if not more — of unpredictable maneuvering on the Senate floor, where Reid and his allies will seek to incorporate changes sought by Democrats and repel attempts by Republicans to defeat the legislation and inflict a significant political defeat on the president.

Reid was releasing his legislation more than a week after the House approved its version of the health care bill on a near party-line vote of 220-215.

According to estimates from the Congressional Budget Office, that House bill, with a price tag of about $1.2 trillion, would result in coverage for tens of millions of uninsured, and provide 96 percent of the eligible population with insurance.

Reid has said he was seeking a less costly measure, but it was not clear whether he would meet or slightly exceed Obama's target of roughly $900 billion over a decade.

Two Senate committees approved different versions of a health care bill earlier in the year, and while Reid has said he would produce a blend of the two proposals, in fact he had a virtual free hand to come up with a plan that could command the 60 votes needed to pass.

Anticipating a major struggle, the White House deputized Interior Secretary Ken Salazar and former Senate Majority Leader Tom Daschle to join Vice President Joe Biden in trying to clear the way for the bill's approval over the next several weeks.

Salazar, a former Colorado senator, is viewed as a bridge to moderate Democrats who are far outnumbered by liberals inside the Democratic caucus.

Daschle was Obama's first choice for secretary of health and human services, a position from which he was to try and oversee the administration's drive to enact health care legislation. He withdrew his nomination when it was disclosed he had not paid more than $120,000 in federal taxes over several years.

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Associated Press writers Ricardo Alonso-Zaldivar and Erica Werner contributed to this report.