President Bush issued the first veto of his five-year-old administration yesterday, rejecting Congress's bid to lift funding restrictions on human embryonic stem cell research and underscoring his party's split on an emotional issue in this fall's elections.

At a White House ceremony where he was joined by children produced from what he called "adopted" frozen embryos, Bush said taxpayers should not support research on surplus embryos at fertility clinics, even if they offer possible medical breakthroughs and are slated for disposal.

The vetoed bill "would support the taking of innocent human life in the hope of finding medical benefits for others," the president said, as babies cooed and cried behind him. "It crosses a moral boundary that our decent society needs to respect." Each child on the stage, he said, "began his or her life as a frozen embryo that was created for in vitro fertilization but remained unused after the fertility treatments were complete. . . . These boys and girls are not spare parts."

Within hours of Bush's announcement, the House, as expected, fell short in a bid to override the veto, extinguishing the issue as a legislative matter this year but not as a political matter. Democrats said voters will penalize GOP candidates for the demise of a popular measure, and predicted the issue could trigger the defeat of Bush allies such as Sen. James M. Talent, who faces a tough reelection battle in Missouri.

"Those families who wake up every morning to face another day with a deadly disease or a disability will not forget this decision by the president to stand in the way of sound science and medical research," said Sen. Richard J. Durbin (D-Ill.).

Some conservatives also criticized the veto. "I am pro-life, but I disagree with the president's decision," said Senate Majority Leader Bill Frist (Tenn.), a heart surgeon who is weighing a 2008 presidential run. "Given the potential of this research and the limitations of the existing [human embryonic stem cell] lines eligible for federally funded research, I think additional lines should be made available."

The House and Senate passed the bill by comfortable margins but not with the two-thirds majorities required to override a veto. The House voted 235 to 193 yesterday to override Bush, falling short of the threshold and negating the need for a Senate override attempt.

Bush did sign a bill, unanimously passed this week by the House and Senate, to ban the creation of human fetuses for the sole purpose of harvesting organs. But the House thwarted prompt passage of another bill he had hoped to sign yesterday. It would have promoted efforts to conduct stem cell research without destroying human embryos. Bush called it "an important piece of legislation," but several Democrats called it a political fig leaf intended to distract attention from his veto of the long-debated funding measure for embryonic stem cells.

Bush has threatened vetoes on numerous issues over the years, but he and the Republican-controlled Congress had always worked out their differences. On stem cells, however, the president drew a sharp line during his first nationally televised address, on Aug. 9, 2001, banning government funding for research using human embryonic stem cell colonies created after that date.

Over the next five years, public sentiment increasingly moved away from him as celebrities such as Nancy Reagan and Christopher Reeve touted the potential that embryonic stem cells offer in treating Alzheimer's disease, Parkinson's disease, diabetes, spinal cord injuries and other conditions. Unlike "adult" stem cells, embryonic cells can replicate themselves and turn into almost any human tissue.

Officials say that about 400,000 frozen embryos are stored at U.S. fertility clinics. The vast majority await disposal because the couples that produced them have completed their pursuit of children and do not want another person to raise their biological child. Bush praised those who "adopt" such embryos, implant them in a woman's womb and bring them to term.

But others said there will be few such adoptions because most couples seeking a child through in vitro fertilization want a genetic connection to that child. "Even with federal funding available to encourage adoption, the number is 128, which makes it conclusive that these 400,000 embryos will either be used for scientific research or thrown away," Sen. Arlen Specter (R-Pa.), a proponent of the bill, said this week.

Bush and his allies say that frozen embryos are tantamount to humans, and therefore are no more appropriate for medical research than are death row inmates. "If this bill were to become law," Bush said yesterday, "American taxpayers would for the first time in our history be compelled to fund the deliberate destruction of human embryos."

Others reject that analysis, saying it would make killers of every couple that produces an unused embryo, and every employee and official who allows fertility clinics to produce and store such embryos.

"If that's murder, how come the president allows that to continue?" asked Sen. Tom Harkin (D-Iowa). "Where is his outrage?" Harkin called the veto "a shameful display of cruelty, hypocrisy and ignorance."