A top Republican senator has backed a bill easing restrictions on embryonic stem cell research, in a move that breaks rank with President George Bush.
Senator Bill Frist says the research is promising
Bill Frist, Senate Majority leader and surgeon, formally announced his decision in the Senate on Friday.
He said he was pro-life but at the same time believed stem cell research "should be encouraged and supported".
President Bush has threatened to veto stem cell legislation already passed in the House of Representatives.
The new legislation would allow expanded financial support for embryonic stem cell research.
Mr Frist's support for the bill puts him at odds with legislation passed by the Bush administration in 2001 restricting human stem cell research.
White House spokesman Scott McClellan played down suggestions that the surgeon's stance would cause a rift with the president.
"The president and Senator Frist had a good discussion last night," he said. "Bush said 'you have to vote with your conscience'."
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
Speaking in the Senate, Mr Frist said: "I am pro-life, I believe human life begins at conception. I also believe that embryonic stem cell research should be encouraged and supported."
He added that "to me it is not just a matter of faith, it is a matter of science ".
Speaking earlier on ABC's Good Morning America programme, Mr Frist said: "This research is promising, embryonic stem cell research as well as adult stem cell research.
"Now is the time to expand the president's policies because it's promising research; but it must be done in a way that is ethically considerate."
Mr Frist added that only stem cells from embryos that "would otherwise be discarded" should be used.
A heart-and-lung transplant surgeon, the senator is seen as a likely presidential candidate when Mr Bush's second term comes to an end in 2008.
His views could put him on a collision course with the White House and the Christian Conservatives whose support he would need if seeking nomination as a presidential candidate.
In 2001, Mr Frist set out his belief in the value of embryonic stem cell research within certain ethical boundaries.
The new bill would ease restrictions on state funding for research carried out using stem cells from embryos left over from in-vitro fertilisation treatments.
Supporters argue that stem cells, which have the unique ability to develop into any kind of cell, offer the prospect of a cure to conditions including cancer and Alzheimer's disease.
Those who oppose the research argue that the process of extracting the stem cells destroys the embryo and so takes a life.