Claiming unstoppable momentum, President Barack Obama is heading to Capitol Hill Saturday to rally House Democrats for a final push on landmark health care legislation ahead of a critical vote that could make-or-break his presidency.

Mr. Obama has put his presidency on the line to gain passage of his top domestic priority in the face of unanimous opposition from Republicans who say the plan amounts to a government takeover of health care that will lead to higher deficits and taxes.

The battle tilted in Mr. Obama’s direction Friday as more Democrats revealed their positions. But with a 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, Mr. Obama decided to make one final, personal appeal to rank—and—file Democrats, arranging a visit to the Capitol Saturday afternoon.

The sweeping legislation, affecting virtually every American and more than a year in the making, would extend coverage to an estimated 32 million uninsured Americans, 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.

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 more low—income Americans.

Under a complex and controversial procedure Democrats have devised, a single vote will likely be held in the House to endorse a bill approved by the Senate last year as well as a second measure with a package of fixes agreed to in negotiations with the White House.

Congressional analysts estimate the cost of the two bills combined would be $940 billion over a decade. The bill would remake one—sixth of the U.S. economy.

The Senate would then use a procedure called reconciliation to pass the fix—it measure that requires only a simple majority of 51 in the 100—member body, avoiding Republican delaying tactics.

Scrambling to gather the 216 votes needed for passage in the House, Democratic leaders and Mr. 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 of Michigan, 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 of Colorado, as she left an evening meeting with House Speaker Nancy Pelosi. 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 of West Virginia, a co—sponsor of Stupak’s resolution.

Yet, the vote count was breaking in Mr. Obama’s favour Friday.

Four more Democrats announced support for the bill after voting against an earlier version that passed last year, bringing the number of switches in favour of the bill to seven. Republicans resorted to unusually personal criticism in their struggle against the bill, calling Rep. Suzanne Kosmas of Florida a “space cadet” for switching her vote to “yes.”

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.

Republican leaders vowed to make Democrats who support the legislation in swing districts pay a political price for supporting the legislation in November’s national election when control of Congress will be at stake.

Republicans said, as they have from the outset, that Democrats were angling for a government takeover of health care. They also said the cost of the bill would be covered by $900 billion in higher taxes and cuts in future Medicare payments. Medicare is the government—run program to provide health care to the elderly.

“This bill requires 10 years of tax increases and 10 years of Medicare cuts just to pay for six years of supposed benefits, many of which don’t even go into effect until 2014,” House Republican leader John Boehner of Ohio said in the Republican’s weekly radio and Internet address. “That’s not reform.”