President George W. Bush plans to cast the first veto of his presidency, possibly as early today, to stop legislation that would expand federally funded embryonic stem cell research.
The US Senate last night approved the legislation, which has also been passed by the House of Representatives.
But neither chamber expects to have the two-thirds majority needed to override Mr Bush's veto.
The stem cell debate has emerged as an issue in several Senate races in the run-up to congressional elections this November and may be a factor in the 2008 presidential contest.
In 2001 Mr Bush allowed scientists to use 78 stem-cell lines then in existence, most of which proved unsuitable for research. But Mr Bush opposes expanded research because it entails destruction of human embryos.
The new bill would allow federally funded research using excess embryos from fertility clinics, which would otherwise be thrown out. Supporters say they should be used to search for cures for illnesses ranging from diabetes to paralysis. White House spokesman Tony Snow said Bush's veto would be "pretty swift."
Lawmakers expect it as soon as today, and Mr Bush has scheduled an afternoon White House event to give his views on the issue.
Groups opposed to abortion rights have praised Mr Bush's stance but some leading researchers and patient-advocacy groups, such as the Christopher Reeve Foundation and the Michael J. Fox Foundation for Parkinson's Research, urged Mr Bush to reconsider.
"This bill allows important research to advance and creates an ethical framework that will ensure it is done appropriately," said Jim Greenwood, head of the Biotechnology Industry Organisation.
Mr Bush has never used his veto and is the first president to complete four years in office without a veto since John Quincy Adams in the 1820s.
He has threatened vetoes before but has refrained after reaching compromises with the Republican-controlled Congress.