WASHINGTON -- President Barack Obama pitched his ambitious plan to both talk radio listeners and his own liberal supporters in the face of steep resistance from opposition Republicans and decreasing support in the polls.

Liberals were on the verge of revolt as Obama refused to say any final deal must include a government-run insurance option, while Republicans pressed their all-but-unified opposition to the White House effort. Obama, who leaves Washington on Friday on vacation, said reason would prevail and it was no time to panic.

"I guarantee you ... we are going to get health care reform done. And I know that there are a lot of people out there who have been hand-wringing, and folks in the press are following every little twist and turn of the legislative process," Obama told a caller to Philadelphia-based radio talk show host Michael Smerconish during a broadcast Thursday from the White House Diplomatic Reception Room.

"You know, passing a big bill like this is always messy."

Obama is struggling to regain the momentum on a comprehensive bill that would extend health coverage to nearly 50 million Americans who lack it and restrain skyrocketing costs. The United States is the only developed nation that does not have a comprehensive national health care plan for all its citizens.

Obama has proposed a system that would include government and private insurers. Republicans say that private insurers would be unable to compete, leaving the country with only a government-run health program. They warn that could leave Americans with little control over their health care.

Opponents of the overhaul have drowned out supporters at lawmakers' town halls around the country this month, and public backing for Obama's effort has slipped in opinion polls.

Congressional Democratic leaders are preparing to go it alone on legislation, although in the Senate bipartisan negotiations continue among a group of three Democrats and three Republicans on the pivotal Finance Committee.

The so-called Gang of Six convened a conference call late Thursday to resume their talks, their first discussion since they dispersed for Congress' August recess, in some cases to be greeted by angry constituents at home.

Top Finance Republican Sen. Chuck Grassley of Iowa, the Republican's key negotiator, said this week that he'd have to take his constituents' concerns into consideration in continuing talks. But after members of the group spoke for about an 1 1/2 hours Thursday, senators said they remained committed to producing a bipartisan bill and planned to continue work with an increased focus on affordability and costs.

"Tonight was a productive conversation. We discussed our progress and remain committed to continuing our path toward a bipartisan health care reform bill," said Finance Chairman Max Baucus, a Democrat.

The group is aiming to finalize legislation by Sept. 15, and Baucus said the group will meet again before the Senate returns to Washington at the beginning of the month.

On the defensive, Obama is embracing a new role of fact checker-in-chief, trying to correct untrue claims such as that the proposals would provide health care for illegal immigrants, create "death panels" or pay for abortions with taxpayer dollars. Aides say the situation has left Obama exasperated.

"Now, c'mon," a mocking Obama told a cheering crowd late Thursday at a Democratic National Committee appearance designed to re-energize activists who were instrumental in his drive to the presidency. "What we're going to have to do is to cut through the noise and the misinformation."

"I said during the campaign that the best offense against lies is the truth," Obama said. "And so all we can do is just keep on pushing the truth."

Yet for all the gnashing from Republicans and fiscally conservative Democrats, he faces equally tough opposition from lawmakers and activists on the left who insist any overhaul must include a government-run insurance option.

In fact, shortly after his comments Thursday, House Speaker Nancy Pelosi declared the Democratic-controlled House simply won't approve the overhaul without it.

"There's no way I can pass a bill in the House of Representatives without a public option," Pelosi said after a round-table in San Francisco.

Obama told his DNC audience -- as well as thousands watching online and listening by telephone -- that health care was the toughest fight he has faced in office.

"Winning the election is just the start," he said. "Victory in an election wasn't the change that we sought."

That election, though, came with his promise of the government insurance option, a provision that Obama's team now calls "preferred" but not mandatory. During both his Thursday appearances, Obama declined to call it a deal breaker.

He told Smerconish that "the press got excited and some folks on the left got a little excited" when he and top administration aides last weekend made statements indicating that a publicly run health insurance option was just one of several alternatives.

Since then, Obama has faced increasing criticism from his left flank.

While the White House insists Obama is still looking for Republican support for a comprehensive health care bill, Democrats privately are preparing a one-party push, which they feel is all but inevitable. Polls show slippage in support for the president's approach, although respondents express even less confidence in Republicans' handling of health care.