US President George W. Bush cast his first veto last night to block legislation to expand embryonic stem cell research, putting him at odds with top scientists, most Americans and some fellow Republicans.
"It crosses a moral boundary that our decent society needs to respect, so I vetoed it," Mr Bush said of the research that involves tiny human embryos.
President Bush
The US House failed to muster the two-thirds vote necessary to override the veto, Mr Bush's first in more than five years in the White House. The vote was 235-193.
The issue has become ethically and politically volatile because extracting the cells entails destruction of an embryo.
Mr Bush believes that is destroying a life. His opponents say the research, which would be done only on excess embryos from fertility treatments that would otherwise be destroyed, is potentially life-saving.
The debate has become an issue in several of this November's Senate races and it may factor in the 2008 presidential contest.
The veto fulfils a Bush promise made to socially conservative supporters whose votes his Republican Party will need in November to help keep control of Congress.
But Democrats, citing opinion polls showing that most Americans support the research that could lead to new treatments for conditions ranging from diabetes to paralysis, said that Bush's stance may alienate centrist voters.
In 2001, Mr Bush allowed federally funded research on 78 stem-cell lines already in existence, most of which proved inadequate for research. The bill he vetoed would have expanded the number of stem cell lines available for federally funded science.
Mr Bush advocated searching for stem-cell techniques that advanced science in an "ethical and morally responsible way." "I made it clear to the Congress that I will not allow our nation to cross this moral line," he told a White House audience including women who had children after adopting and gestating "spare" embryos from fertility clinics.
"I felt like crossing this line would be a mistake and once crossed we would find it almost impossible to turn back."
Anti-abortion groups have praised Mr Bush's stance, and his allies said the proponents of embryonic stem cell research exaggerated its potential.