President Bush is expected to cast his first veto on Wednesday, one day after the Senate passed a bill, 63-37, allowing federal funding for embryonic stem cell research.
"The simple answer is he thinks murder's wrong," White House spokesman Tony Snow said on Tuesday. "The president is not going to get on the slippery slope of taking something living and making it dead for the purposes of scientific research."
Snow said the president had issued 141 veto threats during his time in the White House, often against spending increases for domestic programs. None came to fruition after deals were cut with lawmakers to avoid that outcome. This was the first time no deal could be cut, Snow said.
Fellow Republicans Nancy Reagan and California Gov. Arnold Schwarzenegger had pleaded with Bush to sign the legislation, which would allow the National Institutes of Health to use stem cells from discarded embryos to look for cures to an array of illnesses from Alzheimer's disease to paralysis.
"Mr. President, I urge you not to make the first veto of your presidency one that turns America backwards on the path of scientific progress and limits the promise of medical miracles for generations to come," Schwarzenegger said.
"With this important vote in favor of embryonic stem cell research (H.R. 810), the pleas of so many suffering families have finally been heard," Reagan, the former first lady, said of the Senate vote. "Time is short, and life is precious, and I hope this promising research can now move forward."
The House and Senate would need a two-thirds majority to override the veto, which they do not have. The House passed the bill in May 2005 on a 238-194 vote. House GOP leaders planned an override vote as early as Wednesday evening, confident that it will fail.
The Senate also passed two related measures that Bush wanted to sign into law, but the House rejected one of them in late voting Tuesday.
The one that passed bans "fetal farming," the possibility of growing and aborting fetuses for research. The one that failed in the House needed a two-thirds vote to pass. It came short by 12 votes. It would have encouraged stem cell research using cells from sources other than embryos in an effort to cure diseases and treat injuries.
The alternate stem cell sources bill "will amend the Public Health Service Act to require NIH to conduct and support basic and applied research to develop techniques for the isolation, derivation, production, or testing of stem cells that have pluripotent or embryonic-like qualities," Rep. Roscoe Bartlett, R-Md., sponsor of the failed House measure, said before the vote.
"To me and millions of other Americans, deliberately taking the lives of innocent human embryos is an unacceptable trade-off. A number of scientists have now proven what I have argued for the past five years. It is scientifically unnecessary to destroy human embryos to obtain pluripotent stem cells," Bartlett said.
Enactment of the bill to encourage research on adult stem cells enables Bush and other opponents of embryonic stem cell studies to say they, nonetheless, support stem cell science.
"The president is not opposed to stem cell research, he's all for it," Snow said.
If scientists could learn to control stem cells and coax them into becoming specific types on demand, they could grow replacements for damaged tissue.
Opponents of federal funding for embryonic stem cell research say studies on cells derived from adults and umbilical cords is more advanced, less controversial and more deserving of federal funding.
How fast the science for both types of stem cell research proceeds depends on how much money the federal government is willing to spend, and for which kind. Supporters of the embryonic stem cell bill say the engine of public funding would greatly accelerate cures and treatments.
Polls show as much as 70 percent public support for embryonic stem cell research.
"There has been an upsurge of demand," said Sen. Hillary Clinton, D-N.Y. Support for the legislation "has crossed every line we could imagine, certainly partisan lines, ethnic, racial, geographic lines."
Actress Mary Tyler Moore appeared with Frist during the day, saying she was very disappointed by Bush's stance.
"This is an intelligent human being with a heart, and I don't see how much longer he can deny those aspects of himself," she said.
The Associated Press contributed to this report.