WASHINGTON - A political storm gathered yesterday over efforts to restrict coverage for abortion in the health-care overhaul, threatening to swamp Democrats' efforts to pass President Obama's signature legislative initiative by year's end.

As the Senate takes up its version of the legislation, abortion opponents there are seeking tougher curbs at least in line with those just approved in the House.

Sen. Ben Nelson (D., Neb.) said yesterday he could not support a bill unless it clearly prohibited federal dollars from going to pay for abortions. He said he was weighing options, including an amendment similar to the one passed by the House over the weekend.

"I want to make sure something comparable . . . is in there," Nelson said.

The House restrictions, offered in an amendment by Rep. Bart Stupak (D., Mich.) and Joseph R. Pitts (R., Pa.), were the price that Speaker Nancy Pelosi - who supports abortion rights - paid to get a health-care bill passed, on a narrow 220-215 vote.

But it has prompted a backlash from liberals at the core of her party.

Abortion-rights supporters in the House were circulating a letter addressed to Pelosi, threatening to vote against a final bill that restricts access to abortion coverage. At least 40 lawmakers had signed on by early yesterday - enough to block passage.

They are likely to have help in the Senate from two Republican women who support abortion rights, Sens. Olympia J. Snowe and Susan Collins of Maine. Collins indicated yesterday that she thought the House went too far.

"I think the Senate Finance Committee did a good job of putting up a fire wall that would prevent federal funds from being used for abortion," she said. "Generally, I prefer the Senate approach."

Although House liberals voted for the bill with the amendment to keep the process moving forward, Rep. Diana DeGette (D., Colo.) said: "There's going to be a firestorm here. Women are going to realize that a Democratic-controlled House has passed legislation that would prohibit women paying for abortions with their own funds. . . . We're not going to let this into law."

The other side in the abortion dispute also has more than enough votes to kill any final overhaul package.

Obama said the legislation needed to find a balance.

"I want to make sure that the provision that emerges meets that test - that we are not in some way sneaking in funding for abortions, but, on the other hand, that we're not restricting women's insurance choices," Obama said in an interview with ABC News.

Senate Democrats will need Nelson's vote - and those of at least a half-dozen other abortion opponents in their caucus.

They will face a grueling debate against Republicans unified in opposition to the health-care overhaul.

"This is a very important issue to Sen. Nelson, and it is highly unlikely he would support a bill that doesn't clearly prohibit federal dollars from going to abortion," said his spokesman, Jake Thompson.

An intraparty fight over abortion is the last thing that Majority Leader Harry Reid (D., Nev.) needs.

Reid is already facing a revolt among Democratic moderates over the government-sponsored health plan that liberals want to incorporate in the legislation as a competitor to private insurance companies.

Reid, who is himself opposed to abortion, will have to confront the issue directly as he puts together a Democratic bill for floor consideration. The committee-passed Senate versions differ on abortion, but none would go as far as the amendment passed by the House.

The House bill would bar the new government insurance plan from covering abortions, except in cases of rape, incest, or the life of the mother being in danger. That's the basic rule currently in federal law.

It would also prohibit health plans that receive federal subsidies in a new insurance marketplace from offering abortion coverage. Insurers, however, could sell separate coverage for abortion, which individuals would have to purchase entirely with their own money.

At issue is a sharp disagreement over how current federal restrictions on abortion funding should apply to what would be a new stream of federal funding to help the uninsured gain coverage.

Abortion opponents have sought to impose the same restrictions that now apply to the federal employee health plan, military health care, and Medicaid, the federal-state health program for the poor.

Abortion-rights supporters say such an approach would threaten women's right to a legal medical procedure already widely covered by private insurance.

The Senate Health Committee bill is largely silent on abortion, a stance that abortion opponents interpret as permitting coverage by private insurance plans that would receive federal subsidies.

The Senate Finance Committee bill attempts to craft a compromise, as the House unsuccessfully tried to do before this weekend's vote tightened restrictions.

The Finance plan would require insurance carriers to separate federal subsidy moneys from funds used to provide abortions, and it would prohibit abortion coverage from being included in a minimum-benefits package. It would require state and regional insurance markets to offer one plan covering abortion and one that did not.

Abortion opponents - including U.S. Catholic bishops - rejected a somewhat similar approach in the House, saying that the approach of keeping federal funds separate amounted to little more than an accounting gimmick.

Visiting Seattle's Swedish Medical Center yesterday, Pelosi said the antiabortion language "would have been in the bill one way or another," so bill backers decided it would be better to include it as an amendment so it would not jeopardize the entire measure.

She said she would continue seeking common ground with abortion opponents.