In a presidency marked by partisan polarization, this may go down as the week of the ascendant moderates - first on judicial nominees and now on embryonic stem cell research.
The stem-cell legislation, passed by the House with a bipartisan majority and headed for likely passage in the Senate later this spring, would expand federal funding for research, a position President Bush vehemently opposes. He has promised his first presidential veto, after an extraordinary stretch without wielding that pen.
And even though Bush has the strong backing of religious- conservative leaders, who believe human-embryo research entails the taking of a human life, a majority of the public - including some conservatives in the House and Senate - believe there is a higher moral value at stake: that this research could someday improve or save the lives of people with diseases the medical world considers incurable.
Bush, who held an event in the White House on Tuesday with children who had been adopted as frozen embryos, has laid down his marker. This issue is a cornerstone in his campaign to promote a "culture of life."
Some moderates are hoping to draw the president into a compromise, given that the margin of House passage, 238 to 194, fell well short of the two-thirds majority needed to override a veto. One possible compromise floated by moderate House leaders would be to push forward the cutoff date for eligible stem-cell lines. In August 2001, when Bush announced federal funding for stem-cell research, he said only existing lines could be used. Many of those have proved unusable.
But social conservative leaders flatly oppose any accommodations that would allow federal funds to be used in the destruction of additional embryos.
"I don't see any reason for the president to compromise," says Gary Bauer, president of the group American Values. "Under the current law, any scientist, medical company, or lab - anyone - can do research involving embryonic stem cells. The only thing we're arguing is whether Americans who are deeply morally offended by that research are going to be required to pay for it."
For Bush, much is at stake in a second and final term that he began with high ambitions but has so far been short on results. On his No. 1 priority, Social Security reform, some religious conservatives are on the fence over the radical changes the president is proposing, such as the introduction of private accounts. In addition, there are times when social conservatives are not fully satisfied with Bush's approach to their issues, such as his initial hesitation about publicly condemning the Massachusetts gay-rights ruling.
"Whenever you get a chance, given that there will be times when you have to irritate the base, it doesn't hurt to shore it up again," says Bruce Buchanan, a presidential scholar at the University of Texas, Austin.
What's clear is that the public posturing over embryonic stem-cell research, on both sides of the debate, will not end anytime soon. At his White House event on Tuesday, featuring 21 families whose children had developed from adopted embryos, the president sought to present an alternative to the destruction of unneeded frozen embryos from fertility clinics. In 2002, Bush signed legislation granting $1 million a year for the Department of Health and Human Services to promote adoption of embryos.
The Snowflakes Frozen Embryo Adoption Program, run by Nightlight Christian Adoptions of Fullerton, Calif., says 81 children have been born through its services, with embryos given up for adoption by the biological parents. It is not known how many other children have been born through this kind of adoption, but the Snowflake program, which began in 1997, is the first of its kind.
Overall, the commonly used figure for the total number of frozen embryos being held in fertility clinics is 400,000.
But many of those are deemed "degraded," and therefore hold low potential for resulting in a live birth. There's also the matter of how the biological parents want any extra frozen embryos to be used, after completion of fertility treatments. Some parents donate the unused embryos for research that is privately funded; others ask that they be discarded. Some prefer that such embryos not be adopted by infertile couples, because they are uncomfortable with the idea of a genetically related child in the world that they are not raising.
"It can be difficult for some couples," says Sean Tipton, a spokesman for the American Society for Reproductive Medicine. "Some disagree, and end up doing nothing."
Of those embryos that are donated for scientific study, much of the research is done on behalf of in-vitro fertility clinics, as they seek to make advances in their techniques, such as figuring out which embryos have the greatest potential for producing a healthy live birth.
According to Mr. Tipton, it is already possible to tell in some cases when an embryo will produce a child with a particular health problem, such as sickle cell anemia. In cases like that, he says, it would be useful to be able to conduct research with such embryos.