WASHINGTON (AFP) - President Barack Obama on Thursday mounted an 11th-hour push behind his top domestic priority, urging lawmakers facing a critical weekend vote on remaking US health care to "pass this reform."

Obama hoped to win over waverers and stiffen supporters' spines in the House of Representatives, where his top Democratic allies publicly predicted victory in a vote expected Saturday -- even as they privately wrangled for support.

Asked whether the sweeping proposal's backers had the 218 votes needed to pass in the 435-seat chamber, Democratic House Speaker Nancy Pelosi told reporters: "We will."

And Pelosi played down the possibility that action in the Senate, where the overhaul effort has bogged down, could slip into 2010, saying: "I don't think that's the case. I think we can do all this before Christmas."

Obama, who planned to meet with Democrats during a rare visit to Congress on Friday, made an unusual appearance at his spokesman's daily briefing to trumpet endorsements of the embattled legislation by two powerful non-partisan groups.

Obama said he was "extraordinarily pleased and grateful" for support from the American Association of Retired Persons (AARP), which represents some 40 million people, and the American Medical Association (AMA) doctors' group.

"I urge Congress to listen to AARP, listen to the AMA, and pass this reform for hundreds of millions of Americans who will benefit from it," said the US president who wants to sign the measure into law this year.

He spoke as several thousand shouting and sign-waving protesters denounced the Democratic approach, backed by leading Republicans who vowed to oppose the legislation.

"I will guarantee you that we are committed to making sure that not one Republican will vote for this bill," said Representative Eric Cantor, the number two Republican in the House.

Some of the demonstrators later fanned out to congressional office buildings, looking to corral lawmakers and vowing to tear up the 2,200-page legislation in front of Pelosi's office.

Obama said the endorsements from people "who know our health care system best" disproved frequent Republican arguments that the overhaul will lead to a government takeover of Americans' personal medical decisions.

"They would not be supporting it if they really believed that it would lead to government bureaucrats making decisions that are best left to doctors. They would not be with us if they believed that reform would in any way damage the critical and sacred doctor-patient relationship," he said.

The White House's de facto political arm, Organizing For America, pushed Obama backers to call lawmakers on behalf of the bill, urging: "With the finish line on health reform in sight, we need your voice more than ever before."

Even if Democrats muscle their majority into passing the measure, it still faces an uphill battle in the 100-seat Senate, where 60 votes are needed to ensure the ability to break parliamentary delaying tactics and pass it.

Senate Democratic Majority Leader Harry Reid earlier this week declined to repeat predictions that the health care overhaul will be complete in 2009, fueling speculation that the effort could slide to next year.

That would put the issue front-and-center in the 2010 mid-term elections, when one third of the Senate, the entire House of Representatives, and many US governorships are up for grabs.

Pelosi dismissed talk that Democrats should worry about the results of four elections held Tuesday, which some analysts have read as a warning to swing-vote Democrats wavering on the health care bill.

"Tuesday night, we won two more votes for health care. I'm very proud of that because in both of those campaigns, health care was the issue," she said, referring to Democrats elected in House races in New York and California.