In a show of confidence, House Speaker Nancy Pelosi emerged from the final Democratic caucus before the vote wielding a large gavel and leading Democrats across the street to the Capitol for the final series of tallies.
"We are doing this for the American people," Pelosi said.
A protester disagreed and yelled back, "You're doing this to the American people!" Others chanted Pelosi's name and shouted, "Kill the bill! Kill the bill!" Supporters cheered the speaker and other Democrats as they entered the Capitol.
Inside the House chamber, negotiations continued on the floor while overhead in the visitors gallery, a protester hollered, "The people don't want this!" As ushers tried to escort him out, several Republicans stood up on the House floor and cheered.
A few hundred protesters carrying signs opposing the health care overhaul crowded a grassy area near the House side of the Capitol. One sign read, "This bill is anti-American, vote the bums out" while another stated, "Obamacare (equals) death warrant for grandma." Appearing to outnumber Americans flags were banners with the colonial-era slogan "Don't tread on me."
Amid the heat, Democratic leaders said they had only a few votes left to lock in. But none was willing to declare victory - a hesitance Republicans attributed to public controversy over the plan in a difficult election year.
The legislation, affecting virtually every American and more than a year in the making, would extend coverage to an estimated 32 million uninsured, bar insurers from denying coverage on the basis of existing medical conditions, and cut federal deficits by an estimated $138 billion over a decade.
Congressional analysts estimate the cost of the two bills combined would be $940 billion over a decade.
Rep. Bart Stupak said he was one of about a half-dozen anti-abortion Democrats who were working out language for a presidential executive order that would provide more assurances that no public money would be used for elective abortions.
Although House Energy and Commerce Committee Chairman Henry Waxman said his understanding was that Stupak had agreed to vote "yes," Stupak told reporters that no deal had been made.
"I'm a 'no' vote. There's no agreement. There is no agreement," Stupak told reporters on Sunday afternoon. "We were close and we went one step forward and two back."
As the House opened the rare Sunday session, Democrats showed progress toward the crucial threshold. Retiring Rep. Brian Baird announced he would switch his "no" vote to "yes," convinced, he said, that the legislation was better than doing nothing.
A key anti-abortion lawmaker, Rep. Marcy Kaptur stepped from undecided to a "yes" vote because she believes the bill would not allow federal money to be used for elective abortions.
At the White House, Obama surprised his senior staff by showing up at a meeting. He was expected to make and take calls from lawmakers as the session wore on.
Across the Capitol, Senate Democratic Whip Dick Durbin said the Senate would be able to muster the majority required to pass a package of fixes. Republicans warned that passage would mean doom for the Democratic majority on Election Day.
House Democrats were cautious but optimistic that they'd get to or beyond 216 votes.
"There are still members looking at it and trying to make up their minds," House Democratic leader Steny Hoyer, D-Md., said on NBC's "Meet the Press" in the hours before the vote. He added that the holdouts numbered in "the low single digits."
"We think there are going to be 216-plus votes when we call the roll," Hoyer said.
Rep. Debbie Wasserman Schultz the party's deputy whip, also said the votes were not yet in hand, telling "Fox News Sunday" that Democrats were still short of "a hard 216."
Public anger over the rewrite played out in angry protests at the doorstep of the Capitol.
It was unclear whether Obama would agree to issue an executive order about abortion funding. Long-standing federal policy bars U.S. aid for abortions except in cases of rape, incest or when the mother's life is in danger.
"We have received assurances that we will work with the administration to assure that existing law is maintained - not to change it in any way, but to make sure that it applies to this bill," Kaptur said.
Another snag involved widespread distrust among House members that the Senate would be able to pass the "fixes" to the bill. Durbin said on CBS' "Face the Nation" that he has commitments of support from at least a majority of the 100-member chamber, but Democratic leaders have not released a list of supporters.
Republicans warned they will make Democrats pay dearly in the fall elections if the fiercely debated measure becomes law.
With Obama's emotional appeal from Saturday ringing in their ears, House Democratic leaders prepared for three showdown votes expected in the afternoon or evening. First up: a "rule" to establish debate guidelines; on a package of changes to a Senate-passed bill, including deletion of special Medicaid benefits for Nebraska; and on the Senate bill itself, the focus of intense national debate for months.
Democrats need 216 votes to pass each one. With all 178 Republicans and at least two dozen Democrats vowing to vote no, the legislation's fate lay in the hands of the Democrats who remained uncommitted ahead of Sunday's vote.
If Democratic leaders prevail on all three House votes, Obama could sign the Senate version of the bill into law. The bill of "fixes" would go to the Senate under fast-track debate rules, called reconciliation, that would enable Democrats to pass it without facing a Republican filibuster.
Democrats control 59 of the Senate's 100 seats, one vote shy of the number needed to overcome bill-killing filibusters from a united GOP.
House Democrats have long insisted that senators agree to change the bill that the Senate passed on Christmas Eve. Since then, it became deeply unpopular with many Americans, because of the special deal for Nebraska, a new tax on generous employer-provided health plans and other aspects.
