"I believe that there is the potential for significant medical breakthroughs, the realization of a cure for young children with juvenile diabetes, spinal cord injury, people with Alzheimer's and Parkinson's," Travaglini said. "I believe we can put in place, and I'm confident this will be the result of the process, the necessary safeguards to ensure that this type of research is supervised and properly monitored." Dr. George Q. Daley, a stem-cell researcher on one of the Harvard teams, explained at Thursday's news conference that creating stem cells through cloning makes it much easier for scientists to study specific diseases and, perhaps, find cures for individual patients.

Cloning allows researchers to create stem cells with the DNA of patients who suffer a particular disease, giving them a new way to study the development of the disease in the laboratory. In theory, cloning could also allow scientists to create embryonic stem cells, and then replacement tissues, which have the DNA of a patient, meaning that the tissue will not be rejected by the body.

"It creates cells that are identical to the patient you hope to treat," Daley said. "It allows us to move a genetic, or misunderstood disease, into the petri dish so we can study it. And it also gives us unique tools for treating an invidivual patient in an highly effective and safe way."

Travaglini's aides were privately incensed Thursday because they said Romney did not alert them to his plans to announce his position and appeared to upstage Travaglini. The governor's remarks surfaced the day after Travaglini filed his own stem-cell bill, which would give the state's endorsement to more types of stem-cell research than Romney's approach.

There are two types of stem cells: adult stem cells, which are already used to treat some diseases, and embryonic stem cells, which are not used to treat diseases but are easier to work with in the laboratory and are able to become a much wider range of cells. Scientists hope to use stem cells to repair tissue damaged by disease and injury.

Most human embryonic stem cells used today were created from embryos left over from fertility treatments.

Romney said although he supports the use of embryos left over from in vitro fertilization, which might be discarded anyway, he opposes the creation of embryos by cloning. Travaglini filed a bill Wednesday that would allow both, but would ban the creation of cloned human babies. President Bush has barred the use of federal money to create any new batches of human embryonic stem cells, restricting researchers to the use of existing stem cell lines.

Romney's aides said he may propose his own bill, depending on what comes out of the Legislature. If he does file his own bill, it could include criminal and civil penalties for research using cloned human embryos.