WASHINGTON — President Obama said Thursday that he “would love to have more Republicans engaged” in the health care negotiations under way in Congress, but expressed doubt that a bipartisan compromise could be reached because he suspected the party’s leadership was intent on defeating his signature domestic priority.

“I think early on, a decision was made by the Republican leadership that said, ‘Look, let’s not give him a victory,’ ” Mr. Obama said, accusing his rivals of trying to reprise the efforts that went into derailing the Clinton health care agenda.

“There are some folks,” he added, “who are taking a page out of that playbook.”

But one lawmaker who participated Thursday night in a conference call among six Republican and Democratic senators trying to reach bipartisan agreement on a health care plan said the senators agreed to keep working together and asked staff members to focus on making health coverage more affordable while holding down medical costs.

“We are hanging in there,” said Senator Kent Conrad, Democrat of North Dakota, after the telephone conference of almost one hour and 40 minutes. “Is it easy? No. But I tell you, I have never seen a group with a greater sense of purpose than this group.”

As the president prepared to leave for vacation on Friday, he sought to allay concerns about his health care agenda from both sides of the political spectrum, hoping to clear up what he called “a lot of confusion.” He appeared on a conservative-leaning radio talk show and later rallied his own network of supporters in a coordinated White House effort to push forward on remaking the health care system.

“You know,” Mr. Obama said, “passing a big bill like this is always messy.”

The president did not entirely close the door to reaching a bipartisan deal, saying he was “happy to make sensible compromises.” But he added that Democrats would not “give up on the core principle that Americans who don’t have health insurance should get it.”

Mr. Obama’s comments came hours before six members of the Senate Finance Committee were to consult in a conference call on the state of their negotiations toward a bipartisan health care agreement. The leaders of the talks, Senator Max Baucus, Democrat of Montana and chairman of the panel, and the senior Republican, Senator Charles E. Grassley of Iowa, have professed some confidence this week that a deal could be struck, but the tone from Mr. Obama and other Democrats suggested skepticism.

As the bipartisan Finance Committee negotiators prepared to discuss a bill that almost certainly would not include a public insurance plan, Speaker Nancy Pelosi told reporters in San Francisco on Thursday that legislation without such a public plan could not win House approval.

Those involved in the Senate negotiations continued to express confidence that they could ultimately reach an agreement when Congress returned next month, though they said any legislation produced would have to be scaled back from measures that have cleared other committees in the House and the Senate.

Mr. Obama talked by telephone with two of the negotiators, Mr. Conrad and Senator Olympia J. Snowe, Republican of Maine. Ms. Snowe is still seen by administration officials and Senate Democrats as a possible ally on the health care legislation, even if other Republicans are unwilling to back a bill.

Republican Congressional leaders disputed the president’s characterization that they had worked in a coordinated effort to oppose the administration’s health care agenda, saying they had legitimate questions about the cost and scope of the plan.

“Americans are rightly skeptical about the administration’s approach to overhauling everyone’s health care and about the more than $1 trillion price tag,” said Senator Mitch McConnell of Kentucky, the Republican leader. “And Americans are concerned about funding new government programs through massive cuts to Medicare and taxes on small business, which is not the bipartisan approach Americans expected.”

Republicans aside, Mr. Obama had plenty to contend with in his own party.

The president took a handful of questions Thursday on the radio talk show, which originated from the White House and was hosted by Michael Smerconish of Philadelphia. Mr. Obama renewed his support for a public insurance option, seeking to quell the uproar among some Democrats that he was abandoning his position to make the plan more acceptable to moderates.

“The press got excited, and some folks on the left got a little excited,” Mr. Obama said, referring to the news reports this week that the administration was leaning toward health care cooperatives as an alternative to a government-run program to compete with private insurers.

Still, the president added that the public option was merely “one choice of many choices,” adding that no one should be “obligated to go into a public plan.”

Asked by one caller what elements needed to be included in a health care plan, Mr. Obama listed four points: reducing the cost of health care, protecting consumers from insurance abuses, providing affordable coverage to uninsured Americans and not adding to the deficit.

Mr. Smerconish, who attracts conservative-leaning listeners, endorsed Mr. Obama’s candidacy last year and moderated over a session of polite questions that belied the intense anger that had erupted over the airwaves this month. Mr. Obama said he had gone on the show to dispel myths that he said had overshadowed the debate, among them that the health plan would provide coverage to illegal immigrants.

One caller, who said that he had supported Mr. Obama and worked for his campaign, said he sensed that the president’s “knees are buckling a little bit” on his pledge to overhaul the nation’s health care system.

“I’m confident that we’re going to get it done,” Mr. Obama said.

Later, Mr. Obama spoke to a gathering of Organizing for America, the group of activists who supported his candidacy. Many members of the organization, along with other liberal groups, have accused him of wavering on his pledge to include a public insurance option as a central part of his health care plan, but no such concerns were expressed on Thursday as Mr. Obama delivered a pep talk to the group in a meeting at the Democratic National Committee that was broadcast on its Web site.

“The best offense against lies is the truth,” said Mr. Obama, barely mentioning the discord in his own party as he urged his supporters to focus on the true opposition. “All we can do is just keep on pushing the truth.”