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President Bush on Thursday signed into law a bill that would make it a separate crime to kill or harm an unborn child during an assault on the mother.

"As of today, the law of our nation will acknowledge the plain fact that crimes of violence against a pregnant woman often have two victims," Bush said before the signing of the measure.

"The death of an innocent unborn child has too often been treated as a detail in one crime but not a crime in itself," the president said.

The Unborn Victims of Violence Act (search) makes it a crime to harm a fetus during an assault on a pregnant woman. Bush signed the bill, which took five years to get through Congress, in an elaborate Rose Garden ceremony.

Raw Data: Unborn Victims of Violence Act (FindLaw)

The controversial measure, which gained new support following the 2002 murder of a pregnant Laci Peterson (search), is also called "Laci and Conner's law." The California woman was eight months pregnant with a son who was to be named Conner when she disappeared on Christmas Eve. Laci Peterson's husband, Scott Peterson (search), has been charged with murder. California is one of 29 states with fetus protection laws.

"All who knew Laci Peterson have mourned two deaths and the law cannot look away and pretend there was just one," Bush said.

Standing behind Bush in the East Room of the White House were a number of prominent lawmakers and people whose pregnant daughters had been slain. Bush focused on one family in particular: that of Laci Peterson.

"All who knew Laci Peterson have mourned two deaths and the law cannot look away and pretend there is just one," Bush said.

"They have laid to rest their daughter, Laci, a beautiful young woman who was joyfully awaiting the arrival of a new son ... This little soul never saw light but he is loved and he is remembered and his name is forever enjoined with that of his mother."

The bill only applies during commission of federal crimes, such as a terrorist attack or drug-related shooting, and only when it is being committed against the pregnant mother.

The bill defines an "unborn child" as a child in utero, which means "a member of the species homo sapiens, at any stage of development, who is carried in the womb."

That definition angers abortion rights supporters, who say the legislation defines life as starting at the point of conception and therefore confers person-status to the fetus. They add that the law will threaten a woman's right to choose.

"The bill President Bush has signed into law today is yet another example of the misleading tactics that opponents of a woman's right to choose are willing to employ," Vicki Saporta, president of the National Abortion Federation (search), said in a statement.

"The bill's supporters claim they are trying to protect women from violence, yet they refused to allow any real violence prevention measures, or even aid to victims, into the bill."

The bill specifically states that legal abortions are not a crime, but critics fear the way it defines life could provide a legal precedent for those who would outlaw the procedure. The Senate narrowly rejected an amendment by Sen. Dianne Feinstein, D-Calif., that would have imposed the same penalties on assailants without referring to the legal status of a fetus.

"This law rights a blatant and indefensible wrong by establishing what everyone already knows — that these children were here," House Majority Leader Tom DeLay (search), R-Texas, said in a statement. "With the president's signature, common sense and common decency are finally placed into law."

Abortion opponents welcome the law as a step toward more protections for the unborn. Bush said he supported the legislation though he did not think the country was headed toward banning abortions.

Bush has said he opposes abortions except in cases of rape or incest or when pregnancy endangers a woman's life. That position has become a standard line in most of his speeches.

"We stand for a culture of life in which every person counts and every person matters. We will not stand for the treatment of any life as a commodity to be experimented upon, exploited or cloned," the president told GOP donors to his campaign at a fund-raiser in Washington Tuesday night.

Sen. John Kerry (search), D-Mass., Bush's presumed opponent in this fall's election, voted against the bill, along with 34 other Senate Democrats, Independent Sen. James Jeffords of Vermont and Republican Sens. Lincoln Chafee of Rhode Island and Olympia Snowe of Vermont. The bill passed the Senate last week. The House approved the measure in February.

"John Kerry strongly supports making it a federal crime to commit an act of violence against a pregnant woman," said Kerry campaign spokesman David Wade. "He agrees with the vast majority of Americans who want tough punishment for anyone who would commit such heinous crimes and know we can do so without undermining a woman's right to choose."

This is the second legislative victory for abortion opponents in the past five months. Last November, the president signed a bill banning the late-term abortion procedure known as "partial-birth" abortion. The ban is currently being challenged in the courts.

As one of the first acts of his presidency, he reinstated the "Mexico City policy" (search) that bars U.S. money from international groups that support abortion, even with their own money, through direct services, counseling or lobbying activities.

He has signed legislation that bans certain late-term abortions and that amends legal definitions of "person," "human being," "child" and "individual" to include any fetus that survives an abortion.

He has increased federal support for abstinence education, adoption and crisis pregnancy programs; placed severe restrictions on federal funding for human embryonic stem cell research to only a few existing cell lines; and extended state health coverage to "unborn children."

Fox News' Wendell Goler and The Associated Press contributed to this report.