WASHINGTON (CNN) -- President Bush on Wednesday vetoed the embryonic stem-cell research bill, his first veto in 5 1/2 years in office.
"It crosses a moral boundary that our decent society needs to respect, so I vetoed it," Bush told backers at a White House event.
House Republican leaders have said they would try for an override vote on the measure, but it's unlikely to pass, lacking the two-thirds majority needed in each chamber.
In August 2001, Bush announced that his administration would allow federal funding only for research on about 60 stem-cell lines that existed at the time. Researchers have since found that many of those lines are contaminated and unusable for research.
"In 2001, I spoke to the American people and set forth a new policy on stem-cell research that struck a balance between the needs of science and the demands of conscious," Bush said.
The president's decision comes a day after the Senate voted 63-37 to loosen Bush's ban on federal funding for embryonic stem-cell research. (Watch how the issue pits Bush against some Republicans -- 1:30)
The measure, which the House of Representatives passed 238-194 in May, allows couples who have had embryos frozen for fertility treatments to donate them to researchers rather than let them be destroyed.
The issue has split the Republican Party, with Bush siding with the Catholic Church and social conservatives against the GOP's more moderate voices. Specter, who recently survived a brush with cancer, was joined by Majority Leader Bill Frist, R-Tennessee, a physician who argued that Bush's policy is too restrictive.
Other supporters included former first lady Nancy Reagan, whose husband's long battle with Alzheimer's disease helped draw attention to the issue.
"Time is short, and life is precious," Reagan said before the veto, "and I hope this promising research can now move forward."
Senate Minority Leader Harry Reid, D-Nevada, had urged Bush to listen to moderate Republicans and "Americans crying for help" and stay his veto threat.
Scientists believe stem cells offer the possibility of a renewable source of replacement cells and tissues to treat afflictions such as Parkinson's and Alzheimer's diseases, spinal cord injuries, diabetes, strokes and burns.
Opponents objected to the destruction of human embryos to extract stem cells and warned that lifting Bush's restrictions would lead to the cloning of human embryos for research purposes.
"Each and every one of us began as an embryo," said Sen. David Vitter, R-Louisiana. "Therefore, I firmly believe that [neither] Congress, independent researchers nor any human being should be allowed, in effect, to play God by determining that one life is more valuable than another."
But Sen. Orrin Hatch, R-Utah, a staunch opponent of abortion, said the bill is promotes life by encouraging research.
"I believe we are aiding the living, which is one of the most pro-life positions you can take," Hatch said.
Opponents argue that other alternatives, such as adult stem cells, are available. Two companion bills -- one to promote alternative means of developing stem-cell lines from sources such as placental blood and another to ban the commercial production of human fetal tissue, also known as "fetal farming" -- passed the Senate 100-0.
On Tuesday evening, the House approved the "fetal farming" bill 425-0 but didn't pass the measure promoting alternative stem-cell sources when backers failed to achieve the two-thirds majority that House rules required. The vote on the alternative-sources bill was 273-154.
A House GOP aide said that the leadership would bring the funding bill back to the floor at another time under a different set of rules that would require a simple majority to pass the measure.