"I don't know what's going to happen with the politics of this thing," the president said at a rally at George Mason University. "Here's what I do know. I do know that this bill, this legislation is going to be enormously important for America's future. I do know the impact it will have on the millions of Americans who need our help."
The president compared the health care vote to historic legislation that created Society Security and the Civil Rights Act of 1964.
"You know, the naysayers said that Social Security would lead to socialism," Obama said to laughter. "But the men and women of Congress stood fast and created that program that lifted millions out of poverty."
"I know this has been a difficult journey. I know this will be a tough vote," he said. "I don't know how passing health care will play politically, but I know it's right. We are going to get this done. We are going to make history."
Obama has invited the House Democratic caucus to the White House Saturday to discuss the health care bill.
Democratic leaders are only about six votes shy of getting the 216 they need to pass the health care bill in the House, and today they expressed confidence that they would have enough votes when the legislation is brought to the floor this weekend.
"We are excited about the momentum that's developing around the bill," House Speaker Nancy Pelosi, D-Calif., said today.
"We're going to have the votes," the No. 2 Democrat in the House, Rep. Steny Hoyer, D-Md., told reporters.
House Majority Whip James Clyburn, D-S.C., today sent a memo to all House Democrats asking them to declare by 2 p.m. today how they will vote. But even after the deadline passed, many House members were still undecided. Clyburn said they didn't expect everyone to make up their minds until the time came to vote.
Democrats today won key endorsements from the AARP and the American Medical Association, which said in a press release today that "after careful review and consideration," it will support Obama's health care overhaul bill.
"The pending bill is imperfect, but we cannot let the perfect be the enemy of the good when it comes to something as important as the health of Americans," AMA president Dr. J. James Rohack said in a statement. "While the final product is certainly not what we would have devised, we strongly support the parts of this bill that are desperately needed by millions of Americans who are struggling to get or keep health insurance coverage."
The Democratic leadership also nabbed several more "yes" votes today. Rep. Allen Boyd, D-Fla., who voted against the bill last November said he will vote for the health care bill, after studying the revised version and weighing its costs and benefits. Earlier today, freshman congressman John Boccieri, D-Ohio, who also voted against the original House bill last November, announced he would switch and vote for it. At the same time, three other Democrats said they would not vote for the bill, making the momentum unclear.
Several key lawmakers have flipped their vote from a "no" to "yes" in recent days, including Rep. Dennis Kucinich, D-Ohio, Rep. Dale Kildee, D-Mich., Bart Gordon, D-Tenn., and Rep. Betsy Markey, D-Colo.
The roughly dozen Democratic members of Congress who have still not declared whether they wouldl support it want more stringent language on federal funding for abortion. Other liberal members said the bill doesn't go far enough to protect Americans.
After the House passes the health care bill, it has to be voted on in the Senate, so it could be days before it actually lands on President Obama's desk. Pelosi today said that when members of the House vote for the health care bill, they will have assurances from the Senate that it would also pass the same legislation.
The Senate GOP leadership argues the House vote does not signal the end of the health care debate.
"I think any House Democrat that votes for this bill thinking the Senate is going to clean up the mess is delusional," Senate Minority Leader Mitch McConnell, R-Ky., told ABC News' Jonathan Karl today. "I don't think it will pass in the same form."
Others are promising a tough fight in the Senate.
"Well, let me put it to you this way, it's far from going to pass in the Senate," Sen. Orrin Hatch, R-Utah, said in an interview with Bloomberg Television's Al Hunt to air tonight. "I think there'll be at least two major points of order raised, and we'll win on those points of order, which means it has to go back to the House. If those people think they're only going to vote on this once, they're nuts."
With the momentum for the health care debate gaining pace, the president on Thursday again postponed his trip to Australia and Indonesia to June.
The House vote is expected to take place on Sunday, although lawmakers said it could also be pushed back to Monday.
A new poll released today showed Americans are closely divided on the health care overhaul bill. A poll by the Kaiser Family Foundation, conducted in March, found that 46 percent supported the legislation while 42 percent opposed it. Forty-two percent said it's time to take a vote, while 36 percent said they want lawmakers to "to go back to the drawing board and start over again." The rest, two in 10, wanted Congress to drop it entirely.
Of those polled, 45 percent thought they would be better off with reform than without it, and 33 percent said they wouldn't be.
Republicans continue to staunchly oppose the Democrats' health care bill and warned about its consequences.
"If anyone thinks the American people are going to forget this vote, just watch," House Minority Leader John Boehner, R-Ohio, warned today.
Abortion Remains a Thorny Issue
Democratic Rep. Bart Stupak if Michigan said today he's open to a deal but will vote "No" on the health care bill unless changes are made to accommodate his more-stringent language that prevents federal funding for abortions.
Stupak's amendment, which was part of the House bill but failed in the Senate, limits access to abortions for people who receive federal subsidies and those who purchase insurance through a health insurance exchange, a marketplace in which people could shop for and compare insurance plans.
Stupak's more restrictive anti-abortion language passed the House by a vote of 240-194, but it was defeated in the Senate, and it's the Senate health care bill that House Democrats are now amending. Proponents of Stupak argue that language will not be enough to prevent federal funding from going toward those services. But others, such as President Obama, argue that it maintains current law.
"There's still no change," Stupak said on "Good Morning America" today of his plan to vote "no." "My group is holding firm."
But the Michigan Democrat said he is open to a deal on the abortion language if he can get a firm commitment from the Senate.
"There is renewed interest in that piece of legislation that I and a number of us are ready to introduce. It's prepared, everybody's looking at it right now," Stupak said on "GMA." "That's one way maybe -- but we still have to deal with the Senate."
Pelosi wouldn't comment on the negotiations. She said some members of the House "may be" talking about a separate bill with abortion provisions, but the focus right now is to gather the votes for the health care bill.
"This bill is about health care and not about abortion," she told reporters today. "There will be no further changes in the bill."
Another Democrat against the abortion wording in the bill, Rep. Marcy Kaptur, D-Ohio, also said Thursday she is open to a separate bill with the House abortion language.
Health Care Debate Heats Up
The president is reaching out to a number of lawmakers who are undecided about how they will vote.
Even Vice President Joe Biden has been constantly working the phones.
"They say, 'Well Joe, look man, I mean, you know, you guys haven't messaged this very well,'" Biden told ABC News' Jake Tapper in an exclusive interview in Durham, N.C., Thursday. "And, 'you know, this thing has gone on so long.'"
Biden said his response to worried lawmakers is simple: "The proof of the pudding is in the eating."
"I'm telling you, you know, pre-existing [conditions], they're going to be covered. You know we're going to control the insurance companies," the vice president said. "You know people aren't going to lose their health care with their employer like is being advertised."
Biden said once these provisions take effect, and the American people feel the impact, lawmakers who vote "yes" will reap the benefits.
Democrats have been touting a preliminary report from the Congressional Budget Office, which estimated that the health care bill will cost $940 billion over the first 10 years and will reduce the deficit by $138 billion in the same time period
Obama hailed the bill as the "most significant effort to reduce the deficit since the Balanced Budget Act" of the 1990s.
ABC News' Jonathan Karl contributed to this report.