Two US senators, Mr Arlen Specter, a Pennsylvania Republican, and Mr Tom Harkin, an Iowa Democrat, are planning to sponsor legislation that would allow federal funding to extract stem cells for research from embryos that will be destroyed anyway. The bi-partisan effort from a traditional conservative and a traditional liberal could spell trouble for President Bush.
Mr Bush immediately said he would veto any such legislation.
Mr Specter expressed concern that existing stem-cell "lines", or self-sustaining colonies, are not enough to meet current needs for scientific research. "I intend to press for an early Senate vote on my bill," he said.
Mr Bush replied from his ranch in Crawford, Texas, during a signing ceremony for farm legislation. Country music blared from speakers as an audience of more than 40 Texas farmers and their families sat on stacks of hay. The president drove to the podium in a white pick-up truck and emerged in an outfit of cowboy boots, blue jeans and a green open-collar shirt.
He said he would veto any legislation that tried to push federal support for embryonic stem-cell research beyond the parameters he has specified, which confine public financing to stem-cell "lines" already derived from embryos.
He said his speech on the issue last week outlined "what I think is right for America, and any piece of legislation that undermines what I think is right will be vetoed . . . I spent a lot of time on the subject. I laid out the policy I think is right for America. And I'm not going to change my mind."
A USA Today /CNN poll showed that 60 per cent of Americans agreed with Mr Bush's decision. But 52 per cent said the decision was driven by politics; only 36 per cent thought it was based on Mr Bush's deeply held beliefs.
Many lawmakers had hoped that Mr Bush might take a broader approach. In his earlier speech, he articulated such strong sympathy for the view that frozen embryos be used that it seemed, until the end of his remarks, that he might endorse it. But administration officials have said over the past few days that Mr Bush meant to close off that possibility. His emphatic comments this week did just that.
Canadian scientists may have found a way to avoid the controversy over using embryos in human stem-cell research, and offer new treatment for neurological conditions, by isolating stem cells on the skin of adult mice that can grow into brain cells.
A study published on Monday by the Montreal Neurological Institute and McGill University, gives scientists new avenues to pursue in human stem-cell research.
"The hope is that the adult stem cells will provide an alternative approach to using embryonic stem cells, but that still has to be proven," said Mr Karl Fernandes, a researcher. The team found that stem cells isolated in the skin of adult mice can grow into brain cells, fat cells or muscle cells.
"We believe our discovery is important as we have identified an exciting new stem cell from a non-controversial source that holds considerable promise for scientific and therapeutic use," said Ms Freda Miller, the lead researcher.
The team has started experiments with human skin to see if transplants of cells might eventually be possible.
Stem cells can transform themselves into virtually any other cell and could lead to major medical advances, especially in diseases where cells are dying, such as Alzheimer's, Parkinson's, multiple sclerosis or even diabetes.
Additional reporting by Reuters