The Italian couple watched their 4-month-old daughter die of a congenital bone disease last year. Told by a doctor that the disease was passed on genetically at conception, they were determined to take every precaution to avoid a repetition of the painful tragedy.

They looked into undergoing laboratory-assisted fertilization. The resulting embryos, produced outside the womb, could be checked for gene-borne disease before implantation in the mother's uterus.

But under an Italian law passed last year, such a procedure is banned. For one, it is illegal for fertile couples to undergo in vitro fertilization. Moreover, running any health tests on embryos created in a lab is prohibited, even if it is known that the parents carry genes that could pass a fatal disease on to their child.

The prospective parents, construction worker Giuseppe Maltese and his wife, Maria Ditta, are at a loss. They want to have a child but do not want to subject it to the torment their daughter, Erika, went through before she died.

"Those who support this law must never have had children with a grave health problem," Ditta said in an interview near their home in this north-central Italian village. "They must never have seen the martyred little body of their own daughter, who was condemned to death by a disease given her by her parents."

Cases like this one have generated a vociferous fight over Italy's restrictive law on laboratory-assisted pregnancy and stem cell research. Opponents of the law organized a petition drive that forced a referendum on it, to be held Sunday and Monday. Defenders of the measure, which is known as Law 40, say that while imperfect, it is the best compromise available on a touchy and controversial subject. The defenders are urging a voter boycott; a failure to reach a 50 percent turnout would keep Law 40 in force.

What was an internal political matter became an international cause when Pope Benedict XVI endorsed a call by Italian bishops for a voter boycott. Benedict has made the battle against abortion, contraception, in vitro fertilization and stem cell research on embryos a key theme of his young papacy.

"I implore you," he told the Italian bishops on May 30, "to continue the work you have undertaken so that the voice of Catholicism be continuously present in the Italian cultural debate," referring to the referendum. On June 6, he reiterated Vatican teaching against "manipulation" of human embryos, which is "contrary to human love, to the profound vocation of man and woman."

Vatican officials have all but called people like Maltese and Ditta potential murderers for wanting to produce embryos that might be destroyed -- in effect terming it an abortion before implantation. Italian commentators who support Law 40 said people like Maltese and Ditta are trying to produce a perfect child, in the style of Josef Mengele, the Nazi concentration camp doctor.

"Behind this referendum is a project to reinvent man in the laboratory, to transform him into a product to sell like steak or a bomb. Here we return to Nazism," wrote Oriana Fallaci, the Italian author of a series of best-selling books pouring scorn on Islam and asserting the superiority of Western culture.

"We're not trying to create a blue-eyed, blond-haired baby, some representative of a super race," Maltese responded. "Those who attack us say they are for life, but they are for death."

The referendum has shattered political party unity. Some members of Prime Minister Silvio Berlusconi's ruling coalition -- which includes his Forza Italia party, as well as a nationalist faction, a northern regional party and Catholic groups -- have said they will disobey the Vatican's call for a boycott. Berlusconi has yet to declare which way he will vote, or whether he will vote at all.

In the opposition -- other Catholic parties, social democrats and communists -- some members say they will boycott or vote against overturning the law, even though the bloc nominally opposes Law 40. In recent years, referendums have attracted low turnouts, and many observers say Vatican intervention assures this one will fail.

Legislators passed the law after wide publicity over a 60-year-old woman who gave birth after being impregnated by frozen sperm from her late husband, who had been dead for 10 years. Assisted procreation had been virtually unregulated in Italy before Law 40.

The new regulations create numerous prohibitions. Only sterile couples can resort to in vitro fertilization. The law sets a maximum of three eggs that may be fertilized. All three embryos must be implanted simultaneously, and none can be frozen for later use in case the first pregnancy fails. Widows can't be impregnated by the frozen sperm of a dead husband.

No preventive diagnosis can be made on the embryos, and they cannot be destroyed, though the law is unclear about what happens if a mother changes her mind about implantation. Sperm and eggs must originate only with married couples; there can be no third-party donors; and surrogate mothers cannot bring the embryo to term.

The law collides with the rules on legal abortion in Italy. A woman who discovers a congenital disease in the fetus may abort it. But under Law 40, an embryo outside the uterus discovered to be possessing a disease cannot be discarded. Law 40 is one of the most restrictive in vitro measures in Europe, similar to one in effect in Germany.

Spain permits sperm and egg donors, surrogate motherhood, pre-implantation diagnosis for congenital disease and adoption of unused embryos by other couples. Embryos may be frozen for later use. Switzerland is liberal in its laws but forbids surrogate motherhood. France and Britain allow research on stem cells from destroyed embryos.

"We will go to Spain if the referendum fails," Maltese said.

Laws in the United States are also far less stringent. Individual states, which govern in vitro practices, allow everything from in vitro fertilization of single women to surrogate motherhood. Recently, President Bush met with parents of children born from "embryo adoption," babies born from leftover embryos created by in vitro fertilization.

No such reception could take place in Vatican City. Benedict, when he was Cardinal Joseph Ratzinger and head of the Congregation for the Doctrine of the Faith, the Vatican's dogma-enforcing department, wrote a document called Gift of Life. Among other things, the 1987 declaration banned "heterologous" pregnancies: the fertilization and implantation of an embryo produced by either a donated egg or donated sperm or both.

Further, Ratzinger expressed opposition to in vitro fertilization of any sort, including between husband and wife -- what is called the "homologous" option.

"In conformity with the traditional doctrine relating to the goods of marriage and the dignity of the person, the Church remains opposed from the moral point of view to homologous in vitro fertilization," the document says. Because artificial insemination separates the sex act from procreation, it is "prohibited," it states.

Law 40 clashes with the 1987 stand on some forms of in vitro fertilization, and Italian bishops are fighting to ensure against further loosening. The boycott campaign is led by Cardinal Camillo Ruini, who heads the Italian bishops conference and is deputy bishop of Rome. Benedict is officially chief bishop of the city.

"We want science to be at the service of the integral good of man," Ruini said when he announced the boycott drive. Overturning Law 40 would "endanger human and moral foundations of our civilization," he said. Priests are handing out pamphlets titled "Life Can't Be Put to a Vote."

Umberto Veronesi, Italy's leading cancer doctor, took issue with the accusations that proponents of looser laws are evildoers. In the Corriere della Sera newspaper, he described cases in which women might want to freeze an embryo while they are undergoing cancer therapy that might lead to their sterility. "Under Law 40, this can't be done. Doesn't that seem cruel to you?" he asked.

Some backers of Law 40 say the legislation can be altered later and need not be thrown out in its entirety by referendum. That's not good enough for Fabio and Beatrice Caligaris, a Roman couple who underwent in vitro fertilization last year under the new regulations.

After hormonal stimulation, Beatrice produced eight eggs. But under Law 40, only three could be fertilized and implanted. The pregnancy failed, but there are no frozen embryos on hand to try again. She must go through the entire stimulation procedure, a difficult trial for women.

"Is this Christianity or fundamentalism?" Beatrice asked as tears came to her eyes.

Fabio said that they would wait for the referendum results and that if the law stood, they, like Maltese and Ditta, would travel to Spain for further treatment.

Pope Benedict XVI endorsed a call by Italian bishops for a voter boycott. Giuseppe Maltese and Maria Ditta will be unable to test in vitro embryos for a fatal bone disease.