WASHINGTON — Rank-and-file Democrats in Congress remain wary of health care legislation in spite of President Barack Obama's closing argument for overhauling the system, well aware that success is far from assured and political perils abound.

"I think he has succeeded in prying open a window of opportunity, but it's a very narrow window," said first-term Rep. Gerry Connolly, D-Va. "And he and the leadership here had better clamber through that narrow window while they can."

In a speech Wednesday at the White House, Obama called on lawmakers to end a year of legislative struggle and angry public debate and enact legislation ushering in near-universal health coverage for the first time in the country's history. He called for an "up-or-down vote" within weeks under rules denying Republicans the ability to block the bill with a filibuster.

"At stake right now is not just our ability to solve this problem, but our ability to solve any problem," the president said. "And so I ask Congress to finish its work, and I look forward to signing this reform into law."

Appearing before a select audience, many of them wearing white medical coats, Obama firmly rejected calls from Republicans to draft new legislation from scratch.

"I don't see how another year of negotiations would help," he said. "I believe the United States Congress owes the American people a final vote."

Lawmakers were almost finished merging House and Senate versions of sweeping overhaul legislation when a special election last month cost Democrats their filibuster-proof Senate majority, throwing the effort into disarray. Obama is attempting to revive it with one final push, but with Republicans united in opposition, there is no certainty about the outcome.

The lift appears heaviest in the House, where the legislation passed by a narrow 220-215 margin in November. Since then several Democrats have defected or departed, and all 254 who remain are eyeing November midterm elections and a restive electorate clamoring for more jobs and skeptical of the health overhaul effort.

"It's fragile," Rep. Joe Courtney, D-Conn., said of the mood in the House. "It's getting close to the election."

Thirty-nine Democrats voted "no" on the House bill, and House Speaker Nancy Pelosi, D-Calif., will probably need some of those to switch their votes. The legislative package now under consideration has less restrictive language on federal funding of abortion than approved by the House, something that Rep. Bart Stupak, D-Mich., leader of the anti-abortion Democrats, said could cost 10 to 12 votes.

Stupak repeated that assertion Thursday in an interview on ABC's "Good Morning America," saying that "we're not going to vote for the bill with that language in there."

Stupak said, "I want to see health care, but we're not going to bypass some principles that we believe strongly about."

Health and Human Services Secretary Kathleen Sebelius, appearing on the same show, said she hoped that "when the bill is in its final form and people have a chance to look at it, I think they will understand that this bill does not change the status quo on abortion."

"There will be no federal financing of abortion," she said.

One problem is, the House likely would have to accept the Senate version under the procedures that Democratic leaders are planning to employ.

Underscoring the pressure, many of the "no" voters made themselves scarce Wednesday while others said they had to wait to study Obama's plan before stating their position. "I haven't seen the president's proposal so I'm going to look at it," said first-term Rep. Scott Murphy, D-N.Y

Nonetheless, Pelosi vowed to answer the president's call.

"Our families and businesses deserve reform that will create millions of jobs, strengthen Medicare, reduce our deficit and no longer deny care or drop coverage to those who need it most," Pelosi said. "We must act now."

Republicans said Democrats would be sorry.

"Americans do not want a trillion-dollar government takeover of health care stuffed with tax hikes, Medicare cuts and giveaways to Washington special interests," said House Republican leader John Boehner, R-Ohio. "Not today, not tomorrow, not ever."

Obama's endorsement of an up-or-down vote sealed Democrats' intention to move forward under rules allowing for a simple majority vote in the Senate, thereby circumventing Republicans, who now command enough votes to deny Democrats the supermajority normally needed to act. But House Democrats want assurances that if they pass the Senate-approved bill as planned, the Senate will pass a package of changes to fix things House Democrats don't like, such as a special Medicaid deal for Nebraska.

"A big issue for the House is just having suspenders with belts on the plan to ensure we don't get left holding the bag with just the Senate bill by itself," Courtney said.

It will take major efforts by Obama and Democratic leaders in the weeks ahead to put those concerns and others to rest. Obama has already made plans to try to sell the legislation directly to the public in states home to opposed or wavering lawmakers, with visits planned Monday to Philadelphia and Wednesday to St. Louis.

At its core, the legislation still is largely along the lines Obama has long sought. It would extend coverage to tens of millions of uninsured Americans while cracking down on insurance company practices such as denying policies on the basis of pre-existing medical conditions. An insurance exchange would be created in which private companies could sell policies to consumers.

Much of the cost of the legislation, nearly $1 trillion over a decade, would be financed by cuts in future Medicare payments and higher payroll taxes on individuals earning more than $200,000 and couples more than $250,000.

In his latest changes Obama added some Republican ideas raised at last week's bipartisan summit, including renewed efforts on changes in medical malpractice and rooting out waste and fraud from the system.

Associated Press writers David Espo, Ricardo Alonso-Zaldivar, Charles Babington and Alan Fram contributed to this report.