WASHINGTON — Claiming unstoppable momentum but still short of the goal line, President Barack Obama is heading to Capitol Hill to rally House Democrats for a final push on landmark health care legislation.

The battle tilted in Obama's direction Friday as more Democrats revealed their positions. But with hardly a vote to spare, the divisive issue of how to keep federal funds from being used to pay for abortions emerged once again as a potential last-minute obstacle.

With the showdown vote set for Sunday in the House, Obama decided to make one final, personal appeal to rank-and-file Democrats, arranging a visit to the Capitol Saturday afternoon. Republicans, unanimous in opposition to the bill, complained anew about its cost and reach.

Under a complex — and controversial — procedure the Democrats have devised, a single vote probably will be held to send one bill to Obama for his signature and to ship a second, fix-it measure to the Senate for a vote in the next several days.

Democratic leaders and Obama focused last-minute lobbying efforts on two groups of Democrats, 37 who voted against an earlier bill in the House and 40 who voted for it only after first making sure it would include strict abortion limits that now have been modified.

Leaders worked into Friday night attempting to resolve the dispute over abortion. Rep. Bart Stupak, D-Mich., who succeeded last November in inserting strict anti-abortion language into the House bill, hoped to do so again. That prospect angered lawmakers who support abortion rights.

"We're not going to vote for a bill that restricts a woman's right to choose beyond current law," said Rep. Diana DeGette, D-Colo., as she left an evening meeting with House Speaker Nancy Pelosi, D-Calif. Abortion opponents are divided over whether restrictions on taxpayer funding currently in the bill go far enough.

Stupak — with eight Democrats and one Republican as co-sponsors — introduced a resolution Friday that would insert his abortion restrictions as a "correction" to the underlying bill. That would add new complications to the already complex strategy Democrats are pursuing to pass the bill, requiring additional floor votes on a highly charged issue.

Stupak and his backers are hoping they have enough leverage to force the leadership to yield to their demand.

"I think the vote count has always been close," said Rep. Nick Rahall, D-W.Va., a co-sponsor of Stupak's resolution.

Yet, the vote count was breaking in Obama's favor Friday.

Reps. John Boccieri of Ohio, Scott Murphy of New York and Allen Boyd and Suzanne Kosmas of Florida became the latest Democrats to announce support for the bill after voting against an earlier version that passed last year, bringing the number of switches in favor of the bill to seven.

On the other side of the ledger, Reps. Michael Arcuri of New York and Stephen Lynch of Massachusetts became the first Democratic former supporters to announce their intention to oppose the bill. Lynch said he did so despite a telephoned appeal from Vicki Kennedy, whose late husband, Sen. Edward M. Kennedy, championed health care for decades.

Rep. Anh Cao of Louisiana, the only Republican to support the earlier measure, has also announced his opposition.

The sweeping legislation, affecting virtually every American and more than a year in the making, would extend coverage to an estimated 32 million uninsured, forbid insurers to deny coverage on the basis of pre-existing medical conditions and cut federal deficits by an estimated $138 billion over a decade.

Congressional analysts estimate the cost of the two bills combined would be $940 billion over a decade.

For the first time, most Americans would be required to purchase insurance, and they would face penalties if they refused. Billions of dollars would be set aside for subsidies to help families at incomes of up to $88,000 a year afford the cost. And the legislation also provides for an expansion of Medicaid that would give government-paid health care to millions of the poor.

Republicans resorted to unusually personal criticism in their struggle against the bill, calling Kosmas a "space cadet" after she announced her position and labeling Pennsylvania Rep. Jason Altmire a "drama queen" for waiting to announce his opposition.

One day after Democrats released 153 pages of revisions to their bill, they were back at it, responding to fresh concerns from some of the rank and file about disparities in payment levels to Medicare providers in different areas of the country.