WASHINGTON — President Obama urged Democratic senators on Tuesday to persevere in trying to get a bipartisan deal on health care, but left open the possibility that they might have to pass a bill with only Democratic votes if Republicans stood in the way.
At lunch with Democrats at the White House, Mr. Obama vowed to respond to Republican attacks on his plan, which aims to guarantee insurance for all Americans while slowing the explosive growth of health costs.
When Democratic leaders left the meeting, they gave an upbeat account of the lunch, where they joined the president in celebrating his 48th birthday with a chocolate cake.
The Senate majority leader, Harry Reid of Nevada, said there was “absolute unity” among members of the normally fractious Democratic conference.
“Everyone recognizes that we are going to do, if there’s any way humanly possible, a bipartisan bill,” Mr. Reid said. “We don’t want to do a partisan bill, and we hope our Republican colleagues acknowledge that. We’ll continue to work with them as long as we have to.”
Senators said Mr. Obama acknowledged that he and members of his party would face a scorching battle over health care while lawmakers were home for their summer break.
Senator Mary L. Landrieu, Democrat of Louisiana, quoted the president as saying that August would be “a very tough month, in terms of debate.”
The chairman of the Senate Finance Committee, Max Baucus, Democrat of Montana, said lawmakers had turned their attention to sharpening their sound bites.
Public opinion polls show growing doubts about some of Mr. Obama’s health care proposals, and in the last week, several members of Congress have been confronted by constituents, protesters and advocacy groups denouncing Democratic proposals.
Mr. Reid was undaunted.
“In spite of the loud, shrill voices trying to interrupt town hall meetings and just throw a monkey wrench into everything,” he said, “we are going to continue to be positive and work hard.”
“Before year’s end,” Mr. Reid said, “we’re going to get comprehensive health care reform.”
Health care legislation is Mr. Obama’s top domestic priority. Four of the five Congressional committees working on the issue have approved some version of the legislation, but efforts have bogged down in the fifth panel, the Senate Finance Committee, the only one trying to forge a bipartisan proposal.
Mr. Baucus said he did not share the view, expressed by Speaker Nancy Pelosi and some White House officials, that private insurance companies were a major obstacle.
“There are no enemies and villains here,” Mr. Baucus said. “Most Americans want to reform our system. Most companies, industries, want to reform the system because they know we have a lousy system. We have to work together to find out a better solution, which is still a uniquely American solution, which is public and private. We’re not, you know, Great Britain. We’re not Canada. We’re not Netherlands. We’re America.”
Senator Ben Nelson, Democrat of Nebraska, a potential swing vote, said Mr. Obama had urged Democrats to keep negotiating with Republicans.
The president “expressed continuing support for Senator Baucus and working with others to get a bipartisan approach,” Mr. Nelson said.
Senator Ron Wyden, Democrat of Oregon, said Mr. Obama was ready do battle. Mr. Wyden quoted the president as saying, “The White House is not a bad bully pulpit.”
“The president said his first choice is a bipartisan bill, but he also feels strongly this legislation needs to get done this year,” Mr. Wyden reported.
Senate Democratic leaders have been urging the White House to provide more explicit guidance about what Mr. Obama wants to see in a bill. The president apparently did not provide such guidance on Tuesday.
Mr. Baucus said, for example, that the president had not discussed a question that divides Senate Democrats: whether Congress should establish a government-run health plan, to compete with private insurers, or nonprofit insurance cooperatives, owned and run by consumers.
Senator Barbara Boxer, Democrat of California, said Mr. Obama had described the anger at public meetings as “a distraction,” and she quoted him as saying: “We should not be fearful of it. The truth is on our side. If you look at the status quo, it will be very harmful to keep it.”
Centrist Democrats said they were heartened by what they saw as Mr. Obama’s apparent willingness to compromise.
Democratic and liberal groups have been running television advertisements intended to pressure undecided Democrats to support the president’s health plan.
Mr. Nelson, a focus of such advertising, said he was glad to hear Mr. Obama denounce the tactic and affirm that Democrats were all on the same team.
The president “said that it was wrong for Democrats to be running ads against Democrats,” Mr. Nelson said. But the senator said he did not know if the advertisements would stop.