The American Medical Association yesterday endorsed a bill to restrict "partial birth" abortions just as the hotly contested legislation faces a vote in the Senate, possibly today.

The AMA's announcement marks the first time the nation's largest physicians' organization has taken a position on an abortion bill. Only a week ago the group declined to support or oppose the legislation.

The endorsement comes at a critical time. Sen. Rick Santorum (R-Pa.), the bill's Senate sponsor, said he counts 62 "pretty solid" votes for the bill -- five short of the two-thirds needed to override a promised veto by President Clinton. But he said there were six to eight undecided votes that might be swayed by the AMA and claimed that prospects of a veto-proof Senate majority were "improving."

Last week the Senate defeated an alternative drafted by Senate Minority Leader Thomas A. Daschle (D-S.D.) and supported by Clinton that would have banned all late-term abortions but provided exceptions to protect the health as well as life of a woman.

The AMA's board of trustees decided to support the bill after the measure's sponsors agreed to clarifications and procedural safeguards that spell out what procedure is banned and would help protect doctors from overzealous prosecution.

According to Santorum, the changes would make it clear that doctors who are intending to deliver a baby would not be faulted if the disputed procedure suddenly and unexpectedly had to be employed to save the woman's life. The bill also would more narrowly define "partial birth" to exclude other more widely accepted abortion procedures and free doctors from the obligation of showing that no other procedure would suffice under the circumstances.

In addition, Santorum said, any physician accused of performing an illegal abortion would have the right to review by a state medical board before any criminal proceedings, with information obtained by the review to be considered by the court.

With the changes, the bill "impacts only a particular and broadly disfavored -- both by experts and the public -- abortion procedure," said Nancy W. Dickey, chair of the AMA's board of trustees.

James Stacey, AMA spokesman in Washington, said the organization has previously been neutral in fights over abortion legislation and has never before directly endorsed a bill to either expand or restrict abortion rights.

The White House said the AMA statement and Santorum's amendments would not alter Clinton's opposition. "The changes that were made in the bill today do not materially affect the president's concerns, which are to protect the woman from serious health consequences," said deputy press secretary Mary Ellen Glynn.

Glynn pointed out that another leading medical organization, the American College of Obstetricians and Gynecologists, preferred Daschle's language providing for an exception for health risks.

The House passed the bill two months ago with more than enough votes to override a veto. Similar legislation was vetoed last year by Clinton, and the Senate sustained his veto.

Rep. Charles T. Canady (R-Fla.), the bill's chief sponsor in the House, said the AMA's endorsement brings the legislation "within sight" of a veto-proof majority in both houses of Congress.

The legislation would outlaw the procedure called "partial birth" abortion by its opponents, medically known as intact dilation and extraction, during which a surgeon pulls the fetus out of the birth canal feet first and then punctures the head, removes the brain and collapses the skull so the fetus can be removed vaginally.

The procedure could be used only to save the woman's life. No exception is provided to protect her health.

"It is a procedure which is never the only appropriate procedure and has no history in peer reviewed medical literature or in accepted medical practice development," Dickey said in her statement of support for the bill.

"The bill has no impact on a woman's right to choose an abortion consistent with Roe v. Wade (the 1973 Supreme Court decision that legalized abortion). Indeed, the procedure differs materially from other abortion procedures, which remain fully available, in part because it involves the partially delivered body of the fetus which is outside the womb."

There are two main alternatives to intact dilation and evacuation (D&E). One is to remove the fetus in parts, a technique known as "dismemberment D&E."

The other is to kill the fetus by injecting it or the womb with a toxic substance. Labor is then induced, and the woman delivers a stillborn fetus. Those techniques would continue to be legal.

The National Right to Life Committee immediately praised the outcome, saying the clarifications "do not weaken" the thrust of the bill.

But Kate Michelman, president of the National Abortion and Reproductive Rights Action League, bitterly assailed the AMA action in an interview. "It's clear the AMA cares much more about moving their political agenda through a Republican-controlled anti-choice Congress than they do about women's health and women's rights," she said.

Even before the AMA's action, the bill's supporters were encouraged by a statement late last week by Daschle that he might abandon his opposition to the legislation.

Daschle had tried unsuccessfully to win approval of an alternative, which would have banned all late-term abortions (after the fetus is capable of living outside the womb) except when a woman's life is at stake or continued pregnancy would threaten "grievous injury" to her health.

His proposal was defeated, 64 to 36, with many abortion rights backers joining abortion foes in opposition to his proposal.

In a telephone news conference with South Dakota reporters Friday, Daschle, a longtime supporter of abortion rights, said he thought there are "some very legitimate constitutional questions" about the legislation that may very well result in its being overturned by the Supreme Court.

"In the meantime, I think you could make a very strong argument that this abhorrent procedure has to be stopped, regardless of the circumstances," he added.

Daschle said he was consulting constitutional experts on the issue. He declined to comment further yesterday.

The AMA is generally considered the single most influential national group on health policy. But its political clout has been somewhat diluted in recent years by the rise of powerful and well-funded organizations representing the managed care industry, health insurers and other medical groups.

Even so, the AMA remains a major political contributor, giving $2.4 million to candidates for federal office last year; most of it to Republicans. Staff writers David Brown and Judith Havemann contributed to this report.