House Democratic leaders are predicting victory for President Barack Obama's bill designed to overhaul the United States healthcare system.
Passage of a landmark bill would be one of the most significant US legislative triumphs in decades.
The House of Representatives convened in a rare Sunday afternoon session for a series of votes to determine the fate the bill.
Mr Obama has made health care reform the defining issue of his first year in office, setting off a tumultuous debate that has left the country deeply divided. If passed, the reform is likely to be judged alongside the boldest acts of presidents and Congress in domestic affairs.
While national health care has long been a goal of presidents stretching back decades, it has proved elusive, in part because self-reliance and suspicion of a strong central government remain strong in the US
In the hours before the vote, House Democratic leaders were still trying to nail down commitments from a handful of members, some of whom remained concerned about the abortion issue.
Mr Obama, who cancelled a trip to Asia to be on hand for the vote, was spending the day in the White House West Wing for some last-minute lobbying, making and taking calls with politicians.
"There are still members looking at it and trying to make up their minds," House Democratic leader Steny Hoyer said in the hours before the vote. He added that the number of those still undecided were in "the low single digits" and added: "We think there are going to be 216-plus votes when we call the roll."
Republicans attributed the caution to public controversy over the plan, which played out in angry protests at the doorstep of the Capitol during the House of Representatives' rare weekend session.
The last-minute decision-making of politicians gave the House vote a measure of suspense, despite the Democrats' optimism. House Democratic Caucus Chairman John Larson stopped just short of declaring victory, commenting: "We have the votes now - as we speak."
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