Obama's meeting with House Democrats comes on the eve of an unusual Saturday roll call and in the midst of a frenzied effort to round up the 218 votes needed to pass the legislation. Democrats will occupy 258 House seats when Rep.-elect Bill Owens of New York is sworn before the vote, but the size and scope of the health care bill have some Democrats wavering.

Asked Thursday whether she has the votes, House Speaker Nancy Pelosi, D-Calif., said: "We will."

On Thursday, the 1,990-page bill was the subject of last-minute tinkering in the backrooms of Congress and raucous protests in its halls.

After an outdoor rally, opponents of the legislation went to lawmakers' offices. Capitol Hill police said they arrested 12 people outside Pelosi's office, where opponents and supporters staged dueling demonstrations.

The legislation got important last-minute support from the nation's largest association of retirees, AARP, and the American Medical Association. "It's not perfect" but "consistent enough with some of our goals to warrant support," said AMA President James Rohack.

Obama celebrated with an impromptu appearance before White House reporters. "I am extraordinarily pleased and grateful," he said. "We are closer to passing this reform bill than ever before."

Pelosi on Thursday called the bill "historic," and said it will provide coverage for 36 million more Americans.

The bill, which the Congressional Budget Office has said would cost more than $1 trillion over 10 years, requires nearly all Americans to purchase health insurance by 2013 and creates a government-run program, similar to Medicare, to compete with private insurance companies.

To help cover the costs, the legislation would impose a 5.4% income surtax on individuals who earn more than $500,000 a year and married couples making more than $1 million. Still under negotiation: disputes over coverage of immigrants and abortion.

House passage would represent an important milestone for the most ambitious health bill since Medicare in 1965. But the bill faces challenges in the Senate, where Senate Majority Leader Harry Reid of Nevada has said he will not rush to get it done before year's end.

Contributing: David Jackson

Enlarge By Chip Somodevilla, Getty Images Thousands of people rally to protest the health care bill at the U.S. Capitol on Thursday.