A controversial bill expanding federal funds for embryonic stem cell research is set to be passed by the US Senate.
There has been intense lobbying on both sides to argue their case
But the vote will bring a swift response from President George W Bush, who has vowed to wield his veto for the first time to block the legislation.
Opinion polls suggest most Americans back the research, which scientists hope will lead to cures for illnesses like Parkinson's and Alzheimer's.
But Mr Bush has consistently opposed embryonic research on moral grounds.
The Senate vote will come at the end of two days of debates on three separate stem cell bills.
Senators are now giving their final arguments before the voting.
The most controversial bill, the Stem Cell Research Enhancement Act, would scrap limits on federal funding imposed by Mr Bush in 2001.
It has already been passed by the House of Representatives.
In the years since Mr Bush's ban was imposed, pressure has been building for a loosening of restrictions.
Campaigners for stem cell research include prominent Republicans such as Nancy Reagan, whose husband, former President Ronald Reagan, died after a long battle with Alzheimer's.
Monday's debate in the Senate heard impassioned arguments on the issue.
"I lost a beautiful daughter some years ago to heart disease," said Democratic Senator Byron Dorgan.
"I wondered then and I wonder now and I will wonder some long while if there's anything that we could do to unlock the mystery of that devious killer."
George W Bush: 0
Bill Clinton: 38
George HW Bush: 44
Ronald Reagan: 78
FD Roosevelt: 635
Thomas Jefferson: 0
Senator Sam Brownback, a Kansas Republican, argued that it was "immoral to destroy the youngest of human lives" for research.
"Who knows how many human embryos we will have to destroy before any tangible progress is made?" asked Senator Jim Bunning, a Republican from Kentucky.
The White House on Monday reiterated Mr Bush's determination to veto the bill if it is passed.
"The bill would compel all American taxpayers to pay for research that relies on the intentional destruction of human embryos for the derivation of stem cells, overturning the president's policy that funds research without promoting such ongoing destruction," a White House statement said.
Opinion polls suggest almost two-thirds of Americans support the research.
It also seems set to become an issue in November's mid-term congressional elections.
But Mr Bush remains firmly against any change to the law, along with many other conservative Republicans.
Although the bill enjoys support from both Republicans and Democrats, the Senate is unlikely to muster the two-thirds majority needed to override a presidential veto.
Not since Thomas Jefferson has a US president gone this long without using his veto, reports the BBC's James Coomarasamy.
He says the Bush administration has successfully used pro-life issues to mobilise its Republican base, notably in the 2004 presidential election.
It is ironic that one of those issues seems set to result in the president's first veto, he adds.