By James Westhead
BBC News, Washington
US President George W Bush is heading for a major clash with the Senate, and many in his own Republican party, over stem cell research.
Senators passed a bill increasing federal funding for research on human embryos.
President Bush is likely to respond immediately after the Senate vote
But the president, under pressure from religious conservatives, has long promised to use his veto for the first time to block any such bill becoming law.
The Senate passed the bill by a safe margin - but did not muster the 67 votes needed to override a presidential veto.
The entire procedure - the veto and the failed override - could take place in less than 24 hours, as one legislative aide said, "within one news cycle", to minimise the publicity.
Many scientists hope embryonic stem cell research could one day yield cures for illnesses like Parkinson's, diabetes, spinal cord injury and cancer.
Opponents regard experiments involving embryos as a desecration of human life.
In a statement on Monday, the White House emphasised the administration's opposition by underlining the words: "If the bill were presented to the President, he would veto the bill."
The statement said the bill "would compel all American taxpayers to pay for research that relies on the intentional destruction of human embryos".
In 2001, Mr Bush imposed a ban on federal funding for most embryonic stem cell research. Monday's statement ended campaigners hopes that he was persuaded by new science and strong public support for stem cell research.
US public opinion has shifted in favour of the bill, with polls showing three out of four Americans now support embryonic stem cell research.
Opinion has been influenced by campaigners including such prominent Republicans as Nancy Reagan, widow of former President Reagan who died after a long battle with Alzheimer's.
Dana Reeve, the late widow of Superman star Christopher Reeve, also campaigned for the bill along with actor Michael J Fox who has Parkinson's disease.
Embryonic stem cells are so full of scientific promise because they can grow into cells from any part of the body such as muscle tissue, nerves or bones. But they are so controversial because scientists can only obtain them by destroying embryos.
Although this happens at a stage when they are still a clump of cells invisible to the naked eye - less than a week after the egg was fertilised - opponents consider it morally wrong.
There is no need to manufacture new embryos because more than 400,000 remain frozen in fertility clinics across the country, by-products of in-vitro fertilisations that would be destroyed anyway.
In the Senate on Monday, an emotionally-charged debate on the legislation was marked by personal stories of illness, death and hope.
The issue has deeply split Republican ranks.
The Republican majority leader, Bill Frist, a heart surgeon and presidential hopeful significantly allowed the bill to proceed, criticising the restrictions on federal funding of stem cell research.
Ronald Reagan's wife Nancy now campaigns for stem cell research
Another prominent Republican, Judiciary Committee Chairman Arlen Specter, emphasised the bill would use only embryos derived from fertility treatments that would otherwise be discarded.
"A century from now, people will look back in wonderment at how there could be any doubt about using stem cells to save lives and save human suffering," he said.
But Senator Sam Brownback said: "The government should not be in the business of funding ethically troubling research with taxpayer dollars."
He said stem cell research amounted to "treating humans as raw materials".
Two related bills were also passed into law.
The Fetus Farming prohibition Act makes it a crime to use stem cells produced from a pregnancy initiated and terminated specifically to produce tissue while the third bill encourages research on alternative sources of stem cells.
While the whole process is unlikely to lead to significant policy change, it exposes deep divides within the Republican Party during an election year on a right-to-life issue - an area which in the past had united and mobilising the party's support base.
Some analysts suggest it may nevertheless help Republicans in November's mid-term elections by allowing senators to say they took principled stands for or against the research - or in some cases both, depending on their local political circumstances.