By Matthew Davis
BBC News, Washington
In the impassioned debate in the US over embryonic stem cell research, both sides are convinced that lives are at stake.
Mr Bush is in a difficult position
Opponents - led by the president himself - make the case that public money should not be used to support what they call the further destruction of human life.
But supporters argue that such stem cell research could save lives, by providing treatment and even cures for diseases like Parkinson's, Alzheimer's, or childhood diabetes.
The debate touches on biology, morality as well as bitter personal experiences with disease - and it is politically explosive.
Many Catholics and social conservatives in the US - strong supporters of Mr Bush - oppose the destruction of embryos.
But the issue is splitting Republicans in Congress, putting the president - who has promised to veto any bill widening federal funding for the research - in an unenviable position.
'Lives are gifts'
The deeply emotional nature of the debate was illustrated by the appeals made in the run-up to the House of Representatives debate on the issue on Tuesday.
STEM CELL MILESTONES
1960s: Research begins on stem cells taken from adult tissue
1968: Adult stem cells used to treat immune deficient patient
1998: US scientists grow stem cells from human embryos and germ cells, establishing cell lines still in use today
2001: Embryonic stem cell turned into a blood cell
2004: South Korean scientists clone 30 human embryos and develop them over several days
2005: Korean team develops stem cells tailored to match individual patients
CPS:LINK HREF="" ID="4562235" STYLE="rightarrow">Q&A: What are stem cells
Mr Bush made a public appearance to restate his opposition, surrounded by children who would not have been born but for frozen embryos donated by one couple to another.
"These lives are not raw material to be exploited, but gifts," he said.
Meanwhile in Congress, even prominent pro-life Republicans were voting in favour of a bill to widen federal funding for embryonic stem cell research.
"Who can say that prolonging a life is not pro-life?" said Rep Jo Ann Emerson, who said she had a "perfect" pro-life record and whose mother-in-law had died the night before of Alzheimer's disease.
"I must follow my heart on this and cast a vote in favour," she said.
Jim Langevin, (Dem), who was paralysed aged 16 in a gun accident, came to the microphone in his wheelchair.
"Being pro-life also means fighting for policies that will eliminate pain and suffering," he said.
Much of the debate was couched in emotive terms.
House Majority Leader Tom DeLay said that using leftover embryos from fertility clinics amounted to the "dismemberment of living, distinct human beings" because the embryos are destroyed during the research.
At the same time, there are fears that if wider research is not made possible, scientists who depend on federal funding will leave for institutions abroad that offer stem cell research programs, and the US will end up lagging on the international stage.
Earlier this month, South Korean scientists announced they had made stem cells tailored to match the individual for the first time - heralded as a breakthrough in stem cell technology.
There are fears the US is losing ground to other nations
That news only fuelled feelings that - even with the legal private research that is going on in places like California - the US is falling behind.
Dr John Boockvar, a leading stem cell expert at Weill Cornell Medical College, said: "The biggest source of funding for academic research is the government, and we need the government to support research."
He added: "With passage of stem cell legislation, I have the ability to create new stem cell lines, with informed consent from families, with frozen embryos that would have been otherwise discard, to do research on diseases such as Parkinson's."
In 2001, President Bush authorised the use of government money to fund research on stem cells taken from existing embryos. But he banned support for any new stem cell lines.
Dr Boockvar said creation of new lines would enable techniques that were "more effective, advanced, cleaner and purer than what can be done with the 19 lines available now".
"Those are old and contaminated and have changed," he added.
Whether the science can deliver what it promises is also hotly disputed. Religious conservatives say the potential benefits are overstated.
Richard Doerflinger, of the US Conference of Catholic Bishops, said there had been an "appalling degree of ignorance and confusion" in Congress.
"Some [representatives] even said that embryonic stem cells have a proven ability to cure patients and that adult stem cells do not, whereas exactly the opposite is true," Mr Doerflinger said.
"It is always wrong for government to promote the destruction of innocent human life," he added.
"Society must focus its efforts on promoting medical research that all Americans can live with."
The row over stem cell research comes with Mr Bush under pressure over a number of domestic issues.
Negotiating the emotionally charged debate over stem cell research may require him to invest yet more of the political capital he pledged to spend at the start of his second term.