WASHINGTON — Furious liberals on Monday threatened to derail the massive health care overhaul… WASHINGTON — Furious liberals on Monday threatened to derail the massive health care overhaul bill to protest a last-minute deal over insurance coverage of abortions that had secured passage of the legislation in the House.

At least 40 House members pledged not to vote for a final health care bill if the abortion provision survives — endangering the exceptionally fragile Democratic coalition that has kept the bill afloat.

At issue are the insurance policies offered in a new “exchange,” or insurance marketplace, that the legislation would create to help consumers purchase health plans, many using newly created federal subsidies.

The House measure says the federal subsidies cannot be used to buy health policies that cover elective abortion. But abortion rights supporters say this would affect a broad set of consumers, because insurers would likely abandon abortion coverage in all policies offered in the exchange.

The provision “represents an unprecedented and unacceptable restriction on women’s ability to access the full range of reproductive health services to which they are lawfully entitled,” the House members wrote to House Speaker Nancy Pelosi.

It was a tougher line than they had adopted less than 48 hours earlier, when they had, almost to a member, voted to pass the health legislation. The bill cleared the chamber late Saturday night by a mere five votes.

The tumult over abortion now travels to the Senate, where it promises to cause headaches for Democrats still wrestling with fundamental issues of cost, coverage and revenues in its version of the health legislation.

Legislation before the Senate contains looser restrictions on abortion coverage than was approved by the House. But, already, at least one Senate Democrat, Ben Nelson of Nebraska, appears willing to work with abortion rights opponents on language similar to that from the House.

President Barack Obama suggested Monday the House measure might be altered as the legislation moves through Congress, though he did not say he would push for changes himself.

Obama told ABC News the bill should uphold the principle federal money may not be used to subsidize abortions.

“And 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,” he said. “Because one of the pledges I made in that same speech was to say that if you’re happy and satisfied with the insurance that you have, that it’s not going to change.”

The House amendment would allow people buying insurance in the exchange to purchase separate “riders” that would cover abortions. Abortion-rights advocates say few would do so, because few women anticipate an unplanned pregnancy and few insurers are likely to offer such a separate service.

“No one counts on getting an abortion,” said Rachel Laser, a lawyer with Third Way, a Washington think tank that advocates centrist policies.

In 2001, 13 percent of abortions were billed directly to insurance companies, according to the Guttmacher Institute, which studies reproductive health. That figure, however, may understate insurance payments for abortion, because it does not include cases where women paid for the procedure out of pocket and later asked for reimbursement from their insurers.

Dr. Willie Parker, a board member at Physicians for Reproductive Choice and Health, said the amendment could have the greatest impact on women whose underlying health conditions require hospitalization in order for a safe abortion to be performed.

Parker cited an example of a woman with a pregnancy that involves abnormal attachment of the placenta. While a standard abortion may cost just $350, the cost in that situation would range between $3,000 and $4,000.