WASHINGTON — President Obama and Congressional Republicans sparred over health care during an all-day televised forum on Thursday, with Mr. Obama calling on lawmakers “to focus on where we agree” and Republicans repeatedly urging the president to scrap his bill and start over.

Mr. Obama, speaking to lawmakers from his seat at the table they shared, not from a podium or with a teleprompter, used his opening remarks to make the case that reforming the health care system is critical to the nation’s economy. He made no opening bids, but instead called on the two parties to abandon their talking points and engage in a real unscripted discussion, even as he conceded that it might not result in a bridging of the deep philosophical divide between them.

“I don’t know that those gaps can be bridged and it may be that at the end of the day we come out of here saying, ‘Well, we’ve had some honest disagreements,’” the president said, adding, “but I’d like to make sure that this discussion is actually a discussion and not just us trading talking points.”

For more than six hours, including a lunch break, the two sides veered between substance and talking points, touching on cost containment, expanding coverage, medical liability lawsuits and insurance industry regulations. Toward day’s end, Representative Joe Barton, Republican of Texas, said he had never seen “so many members of the House and Senate behave so long so well before so many television cameras.”

Still, it was clear that Republican anger ran deep. One of the liveliest exchanges came when Mr. Obama clashed with his former Republican rival for the White House, Senator John McCain of Arizona, who unleashed a pointed attack on the president for the process that Democrats used to produce the bill — even as Mr. Obama tried to redirect him to talking about its substance.

Mr. McCain pointedly reminded Mr. Obama that both of them had campaigned “promising change in Washington” and that the president had promised to televise his negotiating sessions on C-Span. “I’m glad that more than a year later you are,” the senator said, going on to deride the 2,400 page bill as the being produced “behind closed doors” with “unsavory deals.”

Mr. Obama tried to cut Mr. McCain off. “John, we’re not campaigning anymore, the election is over,” he said.

Mr. McCain laughed. “I’m reminded of that every day.”

In the afternoon, however, after Mr. McCain had complained that elderly Floridians were being spared cuts in one program while other seniors were not, Mr. Obama nodded and acknowledged that his former rival had made a “good point” in calling for a more even distribution of reductions.

The forum, which the White House intended as a back-and-forth between Republicans and Democrats on health care policy, is an extraordinary last-ditch effort by Mr. Obama to revive his health care bill. The White House is betting that the public will tune in and conclude Democrats have better ideas for reforming health care; Republicans are betting the public will favor their ideas.

At the least, it provided the viewers a glimpse of relatively unscripted conversation between the two parties on an issue that has divided them for decades.

“This right here is a dangerous experiment,” the House Republican leader John Boehner of Ohio, told Mr. Obama, gesturing toward a stack of documents — the Democrats’ health care bill. He continued: “A dangerous experiment with the best health care system in the world.”

In his own remarks, Mr. Obama got personal, recounting the story of his mother’s death from ovarian cancer, and the illnesses of his daughters: Malia, 11, who was rushed to the hospital after complaining she couldn’t breathe and, the president said, was diagnosed as having asthma, and Sasha, 8, who had a potentially dangerous case of meningitis as a baby.

And the president tried to turn the tables a bit on Republicans, citing from their own past statements in which they described the need for reform. “John McCain’s talked about how rising health care costs are devastating to middle class families,” he said, referring to his Republican opponent for the presidency, who was sitting in the room. “Chuck,” he said, turning to Senator Charles Grassley, the Iowa Republican, you’ve been working on this a long time.”

But Senator Lamar Alexander, the Tennessee Republican who was selected to give his party’s opening remarks, called on the president to renounce “reconciliation,” the controversial parliamentary maneuver that would enable Democrats to pass the president’s bill with only Democratic votes.

Mr. Alexander called on Mr. Obama and the Democrats “to renounce jamming” the bill “through in a partisan way.”

Mr. Obama made clear he was not going to do so; he told Mr. Alexander that he preferred to “talk about the substance” rather than legislative process — a sign that he is reserving his options to push the bill through Congress using only Democratic votes if he cannot get any from Republicans.

And House Speaker Nancy Pelosi, who spoke after Mr. Alexander, rejected the idea of scrapping the bill and starting over, saying the American people can’t wait for health reform any longer.

“They don’t have time for us to start over,” she said. “Many of them are at the end of the line.”

Throughout the morning, Democrats echoed the president’s theme that there was more agreement than disagreement, and sought to make the case that their bill had incorporated Republican ideas.

Senator Max Baucus, Democrat of Montana and chairman of the Finance Committee, said Democrats welcomed many of the ideas suggested by Senator Alexander and other Republicans.

“We are including most of those provisions, if not all,” Mr Baucus said. He asserted, optimistically, “We are not really that far apart” and “there’s not a lot of difference here.”

But Republicans made clear that they do have differences with the president’s approach. Among the most contentious policy exchanges as the morning went on was a debate between the two parties over the cost of federal regulation for setting detailed standards for insurance nationwide.

Representative Dave Camp, Republican of Michigan, and Senator Jon Kyl of Arizona, the No. 2 Republican in the Senate, cited a report by the Congressional Budget Office, which said that under the Senate bill, the average premium in the individual insurance market would be 10 percent to 13 percent higher than it would be under current law in 2016.

One reason for the higher costs, they said, is that the government would require insurers to offer a richer package of benefits.

“Does Washington know best about the coverage people should have?” Mr. Kyl asked. Or should people be able to choose?

Mr. Obama said federal standards were needed to establish “a baseline level of protection,” setting standards for what he described as “a decent policy.” Such policies may be more costly because they cover more than cheaper policies, he said.

“Yes, I am paying 10 to 13 percent more because instead of buying an apple, I’m getting an orange,” Mr. Obama said. “They’re two different things.”

Polls show that Americans seem to be split on Mr. Obama’s bill. When asked if they support the legislation, a majority of Americans say no. But its individual components — barring insurance companies from refusing coverage based on pre-existing conditions, for example — are popular. Mr. Obama’s task is to remind Americans of what they like in the bill, while beating down the Republicans’ assertion that it is a government takeover of health care.

The event, which is being carried live on C-Span 3 and the cable news networks, was scheduled to last six hours, until 4 p.m., but Mr. Obama extended it. The summit was to cover four major topics: health care costs, insurance reforms, deficit reduction and extending coverage.

For weeks, Republicans have been deriding the event as “political theater,” and to some extent, it is. But it is also an extraordinary moment — a president with his No. 1 legislative priority tantalizingly within reach, waging an unusual live televised conversation with the minority party in a last-ditch attempt to keep it from slipping out of his grasp.

Presidents since Theodore Roosevelt have talked about making health care a right, not a privilege, and Mr. Obama has long said that he would not be the first president to set out such an ambitious goal, but that he intended to be the last. But with Republicans unified in opposition to his plan, his only hope now is convincing Congressional Democrats — and the public — to get behind it.

White House officials blanketed the airwaves this morning, trying to convey Mr. Obama’s message that rising premiums, including the recent announcement of rate hikes in California, make passing an overhaul an imperative. Robert Gibbs, Mr. Obama’s press secretary, said on MSNBC that Democrats could muster the votes to pass the legislation.

“I think there are the votes to pass health care reform because the American people know that the course we’re on is not sustainable,” Mr. Gibbs said. “We know that insurance companies now are mailing out letters for premium increases next year. Those letters in California landed in mailboxes saying that health care was going to go up 39 percent, ten times the rate of health care inflation.”

Even before the session began there were intense negotiations about the optics, and it was clear as lawmakers began filing in that Mr. Obama would indeed have the home court advantage. The White House and Republicans had agreed to an O-shaped table, so that the president and Republicans would be viewed on equal footing.

But Mr. Obama and Vice President Joseph R. Biden Jr. were seated in front of a marble fireplace, flanked by flags, giving it the obvious appearance that they are at the head of the table. At precisely 9:59 a.m., the president walked across the street from the White House to Blair House.

As to what impact the debate will have, it may depend on how many people actually watch. Even some health policy analysts were skeptical; six hours of back-and-forth on cost containment and insurance industry regulation may not exactly make for compelling television.

“A lot of us have jobs,” said Jim Kessler, the vice president for policy at Third Way, a centrist Democratic research group here. “The fact of the matter is that most people are not allowed to watch TV on the job.”

Robert Pear contributed reporting.