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As a crowd of protesters shouted "kill the bill," House Republicans on Thursday rallied opposition against the Democrats' health care legislation, even as President Obama touted two major endorsements from doctors and seniors groups.

The president interrupted the daily press briefing Thursday to note that the American Medical Association and AARP had just endorsed House Democrats' health care plan.

"This is no small endorsement," Obama said of the AARP's backing. "They know it's a good deal for our seniors."

At the same time, Republicans decried the bill on the steps of Congress in a last-ditch bid to derail or at least delay the legislation heading toward a possible vote Saturday.

Thousands of protesters arrived by bus for the rally, which the GOP is calling an emergency "House Call."  The event drew the conservative "tea party" activists but unlike past rallies was officially sanctioned by House Republicans.

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Republicans want those who attend to track down their elected representatives in Congress and put pressure on them to think twice about voting for the more than $1 trillion health care overhaul pushed by House Speaker Nancy Pelosi.

"They're going to listen," said Rep. Michele Bachmann, R-Minn., who originally called for the rally. "The biggest voice in the United States is your voice."

Republicans were formally unveiling their version of a health care reform bill Thursday.

They are looking to reprise the kind of grassroots resistance that boiled up during the August recess at town hall meetings across the country. That resistance seemed to temper Democrats' ambitions for health care reform and just about dash any hope for passing a government-run insurance plan as part of the package. But just a few months later, both the House and Senate have included so-called public options in their bills.

Bachmann said before the rally that the "lessons" of the town halls have been lost.

"Republicans are in the minority, but a minority in Congress plus the American people equals the majority," Rep. Mike Pence, R-Ind., told Fox News Thursday.

Actors Jon Voight and Jon Ratzenberger stirred up the crowd in the early afternoon. Ratzenberger warned about "socialism" creeping in, as Voight decried the "radical Chicago tactics" of the Obama administration. Meanwhile, nine protesters were arrested in a Senate office building for unlawful entry, according to the U.S. Capitol Police.

Republicans say they want to kill the bill with Thursday's rally. But it's unclear how much of an impact the "House Call" can have so late in the process. Pelosi needs to unify Democrats around the package but does not need Republican support given the Democratic majority in the House.

The Republicans have put forward their own version of health care reform as an alternative, but the Congressional Budget Office estimated that their plan -- while keeping premiums down and costing a mere fraction of the cost of the Democrats' bill -- would cover only 3 million more Americans. The Democrats' bill is projected to cover 36 million.

"That's a very big difference," Pelosi told reporters Thursday, criticizing the GOP version for not prohibiting insurance companies from discriminating against those with pre-existing conditions.

Pelosi spokesman Nadeam Elshami blasted the GOP plan in a brief memo Wednesday night.

"Bottom line: Americans lose and insurance companies win under the Republican plan," he said.

The AARP endorsement could help blunt some of the criticism directed at them over planned Medicare cuts. Democratic sources confirmed to Fox News that President Obama plans to meet with House Democrats Friday on Capitol Hill.

But Pelosi and her deputies are still trying to secure Democratic votes, with disputes over federal funding for abortion and other issues causing a split. House Majority Leader Steny Hoyer told wire reporters Thursday that Democrats will have the 218 votes needed when the bill comes up.

Republicans, though, are hoping their wins Tuesday in the Virginia and New Jersey gubernatorial races will give moderate Democrats pause about supporting the 1,900-page bill. They continue to warn that the plan will drive up the cost of private plans and lead to crippling and persistent deficits.

"A lot of Blue Dogs in this country are going to have a lot of 'splaining to do back at home in their districts, where people do not want the health care that's being peddled by this administration," Republican National Committee Chairman Michael Steele told Fox News. "They don't want government control. They don't want a government option."

The Senate, where Democrats have a slighter majority, is not progressing quite as fast. There are emerging doubts over whether the Senate can bring a bill to the floor by the end of the year.