President Bush is expected to sign into a law legislation that would make it a separate offense to harm or kill a fetus if a pregnant woman is attacked in a federal crime. Though critics argue that the legislation is aimed at eroding abortion rights in the United States, it was approved by the Senate and sent to the president Thursday.

Under the bill, which the House passed last month, violence against a pregnant woman would be viewed as two separate crimes: one against her and one against the fetus at any stage of the pregnancy. The assailant would not have to know the woman was pregnant to be prosecuted.

"Our intent is to bring about justice for the mother and for the child, the unborn child as well as the mother," said Senator Mike DeWine, an Ohio Republican, the bill's chief sponsor. "It is to conform with what the vast majority of the American people believe, and that is that when a pregnant woman is assaulted, and she either loses that child or that child is injured, that there are in fact two victims. It is as simple as that."

But opponents argue the legislation is an effort to erode abortion rights, established by the 1973 U.S. Supreme Court ruling known as Roe versus Wade, by recognizing for the first time a fetus or embryo as a person with separate legal rights in the eyes of federal law.

Terry O'Neill of the National Organization for Women noted Senate passage of the bill comes just months after President Bush signed into law a measure banning late term abortions. She said the legislation, known as the Unborn Victims of Violence Act, is another step toward outlawing all abortions:

"The right-wing extremists who are behind this bill eventually will be laying the groundwork for overturning Roe versus Wade," she said.

But supporters say the legislation has nothing to do with abortion.

"Criminals who attack pregnant women are not performing abortions, and their actions are not shielded by the Supreme Court decisions on abortion," said Douglas Johnson, the legislative director of the National Right-To-Life Committee.

The bill was passed after an alternative was defeated. That measure would have allowed prosecutors to file additional charges against a criminal who attacks a pregnant woman but would not have recognized the fetus as a second victim.

"I believe on its face, our substitute amendment is clear. It is definitive," said Senator Dianne Feinstein, a California Democrat, who sponsored the bill that was ultimately voted down. "It will stand the test of time. It will prevent what we hope to prevent, which is the first major law that decides when life begins."

The presumed Democratic nominee for president, Senator John Kerry of Massachusetts, took a break from the campaign trail to return to the Senate floor for the vote.

He voted against the main bill, but had supported Senator Feinstein's measure. That prompted some harsh words from family members of slain pregnant women.

Carol Lyons of Kentucky, who was on hand for the Senate debate and whose pregnant daughter was murdered in January, said although Mr. Kerry may not believe there were two victims in that crime, she says her unborn grandson was just as much a victim as her daughter.

In a written statement, President Bush said, "Pregnant women who have been harmed by violence and their families, know that there are two victims - the mother and the unborn child - and both victims should be protected by federal law."

Opinion polls indicate the legislation enjoys wide public support, particularly in the aftermath of a high-profile murder case in California.

The body of Mrs. Laci Petersen, who was nearly eight months pregnant when she vanished near the end of 2002, washed ashore last April, along with that of her unborn son. Her husband Scott is being tried on double murder charges under California law.

California is one of 29 states that have unborn victim laws.