Senate Democratic leader Harry Reid says his bill will extend cover to 31m
A sharply divided US Senate has begun debating its version of a contentious healthcare bill, a key domestic issue for President Barack Obama.
The legislation aims to extend coverage to tens of millions of uninsured Americans, but faces entrenched opposition from Republicans.
The most controversial elements are a government-run insurance plan and the funding of abortion coverage.
The House of Representatives passed its version of the bill earlier this month.
If the Senate can agree a plan, the two versions will have to be reconciled and passed again by each chamber before they are sent to the White House for approval.
No votes were scheduled for Monday.
'Nothing more important'
The legislation could lead to the biggest changes in US healthcare in decades, if approved.
Senate Democratic leader Harry Reid says it will extend coverage to another 31 million people, or 94% of eligible citizens.
The $848bn (£515bn) 10-year legislation seeks to establish a government-backed "public option" for healthcare coverage to compete with private insurers, but allows states to opt out.
"There's not an issue more important than finishing this legislation," Sen Reid said on the Senate floor, saying weekend sittings could be necessary in December to debate the bill.
In a knife-edge vote this month, he managed to secure the support of all 58 Democratic senators and two independents to overcome Republican opposition and allow a full debate on the bill to proceed on Monday.
30 Nov - Senators return from Thanksgiving recess to debate and propose amendments to the bill
At least three weeks later - Senators vote on final bill
If passed, conference committee set up to reconcile Senate and House bills
Both chambers vote on final version
If passed, President Obama signs bill into law
But he could still face resistance from Democrats Mary Landrieu, Blanche Lincoln and Ben Nelson, and from former Democrat-turned-independent Joe Lieberman.
Senators Landrieu, Lincoln and Lieberman have said they will help Republicans block the bill if it includes a public option.
Sen Nelson has said he will oppose it unless it includes tough new curbs on any government funding for abortions.
Republican Senate Minority Leader Mitch McConnell said the legislation was too expensive.
"The notion that we would even consider spending trillions of dollars we don't have in a way that the majority of Americans don't even want is proof that this healthcare bill is out of touch," he said.
The debate coincided with the publication of a nonpartisan congressional report that both sides seized upon.
The Congressional Budget Office said on Monday that under the new bill insurance premiums would increase for those buying cover independently.
But its report also found buyers would get better cover and six in 10 would see premium payments cut by new federal subsidies.