WASHINGTON — In an emotional session marked by tales of death and hope, the Senate debated yesterday whether the government should pay for new embryonic stem cell research, pushing a measure to do it toward passage and President Bush’s first veto.

“He would veto the bill,” the White House declared in a written statement, underlining the words for emphasis.

That quieted speculation by supporters that Mr. Bush, perhaps persuaded by new science and strong public support for embryonic stem cell research, would reverse course and sign the legislation into law.

Though several Republican Senate leaders support the measure, many GOP lawmakers oppose it, as do conservative voters in a midterm election year.

“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,” the White House said.

Behind the scenes, former first lady Nancy Reagan lobbied lawmakers on the bill’s behalf. Her husband, President Reagan, died in 2004 after a long deterioration from Alzheimer’s disease, one of several illnesses that researchers say stem cell research might eventually cure.

“She is still restless on this issue,” said Senator Kennedy, a Democrat of Massachusetts. “We all know this debate has moved further down the road toward a hopeful conclusion because of her work.”

The chairman of Judiciary Committee, Senator Specter, a Republican of Pennsylvania, a cancer survivor, said his disease is one of many that might be cured sooner with the engine of federal funding behind embryonic stem cell research.

Mr. Specter compared opposition to the bill to historical resistance to research that led to space travel and landmark vaccinations “to show how attitudes at different times in retrospect look foolish, look absolutely ridiculous.”

“There is just no sensible, logical reason why we would not make use of stem cell research,” he said.

The Senate majority leader, Bill Frist, a transplant surgeon whose negotiations permitted the bill to come to the floor after being stalled for a year, attributed the opposition to “fear (that) can also delay scientific advances that are out there before us.”

“We’ve got to work together to allow science to advance” within ethical boundaries, he said.

Where to draw those boundaries is the heart of the debate.

Mr. Bush and his allies believe embryos are nascent human life worth more than the advances they might make through stem cell science and point out that embryonic stem cell research is years away from clinical trials, let alone cures for disease. They rejected arguments that only leftover embryos from fertility clinics would be used.

“Just because the budding lives would not survive does not mean that we should ghoulishly conduct experiments on them,” said Senator Bunning, a Republican of Kentucky.”Who knows how many human embryos we will have to destroy before any tangible progress is made?”

Senator Brownback, a Republican of Kansas, appeared with three children adopted from in vitro fertilization clinics in an effort to put faces on the argument that frozen embryos could have a future other than being subjects of stem cell research.

“It is immoral to destroy the youngest of human lives for research purposes,” Mr. Brownback said.”It is an age-old human debate, whether you allow the stronger to take advantage of the weaker. We have already regretted doing it in the past; we will regret this, too.”

Neither house has demonstrated the two-thirds majority needed to override a presidential veto. Vote counters on both sides said they expected the Senate to pass the bill with at least 60 votes, but they could not predict there would be the required 67 for a veto override.

The House last year fell 50 votes short of a veto-proof margin when it passed the same bill, 238–194.

The Senate was slated to vote this afternoon. Mr. Bush was expected to veto the bill early Wednesday, followed by the House’s override effort.