WASHINGTON – With control of the health care debate slipping from his grasp, President Barack Obama pitched his ambitious plan to both conservative talk radio and his own liberal supporters Thursday — and denied a challenge from one backer that he was "bucklin' a little bit" under Republican criticism.

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 will leave Washington 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 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. 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 bipartisan negotiations continue in the Senate.

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, D-Calif., 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.

"What we've said is that there are a number of components to health care," he told Smerconish, who is generally seen as a conservative, although he endorsed Obama last year and supports abortion rights. "I see nothing wrong with having public option as one choice."

He said "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.

"And even though some White House advisers seem to have forgotten, the reason the public option has become central to reform is simple: We're fed up with the insurance companies and we need real accountability for them," liberal MoveOn.org said in a message sent to its 5 million members while the president was speaking with Smerconish. "They've had decades to fix the problems with our health care system, but they haven't done it."

One caller to Smerconish's program said he sensed the administration was making a misstep.

"I'm getting a little ticked off that it feels like the knees are bucklin' a little bit," said the caller who identified himself as Joe. "You have an overwhelming majority in both the House and the Senate, and you own the whole shooting match. ... It's very frustrating to watch you try and compromise with a lot of these people who aren't willing to compromise with you."

Obama told his audiences he is trying to reach across the aisle to craft a bipartisan plan, even as he blamed Republicans for delay. He peppered his DNC remarks with jokes and jabs at conservatives that had the partisan crowd breaking into applause and laughter.

In response, a spokesman for the No. 2 Republican in the House said he had a question for Obama and his team.

"We would love to know when, exactly — time, date, place — the president or his staff reached out to Republican leaders?" said Brad Dayspring, a spokesman for Rep. Eric Cantor, R-Va.

Republican leaders in May sent Obama a letter outlining the GOP's principles and asking to collaborate.

"And the president's response?" Dayspring said. "Meeting? Nah. Work together? No thanks. Further discussion? Nope. Instead, they went with, 'Thanks for the letter.'"

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.

Former Republican presidential candidate Mitt Romney said Thursday that Obama is struggling to get a health care bill because he has been too deferential to liberals. Romney, who may challenge Obama in 2012, said on CBS' "The Early Show" that "if the president wants to get something done, he needs to put aside the extreme liberal wing of his party."