WASHINGTON, July 19 -- President Bush on Wednesday rejected legislation to expand federally supported embryonic stem cell research, exercising his first veto while putting himself at odds with many members of his own party and what polls say is a majority of the public.
By defying the Republican-controlled Congress, which had sent him legislation that would have overturned research restrictions he imposed five years ago, Mr. Bush re-inserted himself forcefully into a moral, scientific and political debate in which Republicans are increasingly finding common ground with Democrats.
The president laid out his reasoning in a written message to the House of Representatives, then announced his decision in the East Room of the White House, surrounded by babies born through in vitro fertilization using so-called ''adopted embryos.’’
As the infants gurgled and fidgeted in their parents’ arms, Mr. Bush said the bill violated his principles on the sanctity of human life by encouraging the destruction of embryos left over from fertilization procedures. Proponents of the measure have argued that such embryos would be destroyed anyway.
''I felt like crossing this line would be a mistake, and once crossed we would find it almost impossible to turn back,’’ Mr. Bush said. ''Crossing the line would needlessly encourage a conflict between science and ethics that can only do damage to both, and to our nation as a whole.’’
Until Wednesday, Mr. Bush was among just seven presidents -- all of whom served before 1881 -- who had never vetoed a piece of legislation. Four served only partial terms; the other three were John Adams, Thomas Jefferson and John Quincy Adams.
Within hours of the East Room ceremony, the House hurriedly took up a measure to override the veto, but the vote, 235 to 193, fell 51 short of the two-thirds majority required. Fifty-one Republicans, 183 Democrats and 1 independent voted to override, while 4 Democrats joined 179 Republicans in voting to keep the veto intact.
The vote put an end to the bill’s prospects for the year, but not to the stem cell debate, which has escalated into a major issue on Capitol Hill, with Democrats and Republicans alike predicting electoral repercussions in November.
''This is not some wedge issue; this is the soul of America,’’ said Representative Diana DeGette, Democrat of Colorado, who sponsored the bill Mr. Bush vetoed. ''And this is a colossal mistake on the part of the president.’’
But beyond the principles involved, the White House had clearly calculated that it would have been more of a political mistake to sign the bill. Social conservatives, the heart of Mr. Bush’s base, had demanded the president keep his promise to veto any measure that altered the careful compromise he articulated in 2001. With Mr. Bush’s approval ratings hovering at about 40 percent, conservatives are more critical than ever to the president, and he cannot afford to arouse their ire.
''This is a profound moral issue,’’ said Representative Mike Pence, Republican of Indiana, after the White House ceremony. ''The issue is whether or not it is morally right to use the taxpayer dollars of millions of pro-life Americans who find this research morally objectionable.’’
Yet the ground is shifting in the debate, and even Mr. Pence conceded that opponents of the research were ‘’losing the argument with the American people.’’ Republicans, even those like Mr. Bush who oppose abortion, are wrestling with whether embryos that are no bigger than a typographical period but regarded by some as human beings should be destroyed to save lives.
The issue reflects the complex nature of the politics of abortion and medical research in the United States today and is in some ways the flip side of the Democrats’ quandary over abortion. Just as medical advances like ultrasound imaging have spurred greater opposition to abortion, leading some Democrats to recalibrate their views, the promise of embryonic stem cell research has pushed some Republicans toward positions in which black-and-white beliefs about the sanctity of life have given way to more nuanced and ethically complex stances.
As baby boomers have aged, demanding the best medical treatments for themselves and their elderly parents, the public clamor for stem cell research has grown more intense. According to the Pew Research Center, a nonpartisan polling organization that tracks the issue, roughly two-thirds of all Democrats and independents favor embryonic stem cell research, while nearly half of all Republicans do.
That leaves Mr. Bush -- who has not used his veto partly because Republicans have controlled both houses of Congress for nearly all of his presidency -- at odds with many leaders of his own party. They include staunch abortion opponents like Senators Orrin G. Hatch of Utah, Gordon H. Smith of Oregon and the Senate majority leader, Bill Frist of Tennessee. Already, some Republicans who opposed Mr. Bush on the stem cell issue are looking to the presidential elections of 2008.
''When there’s another election, another chapter of democracy opens,’’ Mr. Smith said in an interview. “Most of the candidates who have a shot at winning are in favor of stem cell research. This represents a delay en route, but I know where we’re going, and it’s where the American people want to go.’’
As the White House prepared for the East Room ceremony, advocates for patients who support stem cell research flooded the switchboard with calls urging Mr. Bush not to veto the bill.
''We were really hoping, because so many of the American people supported this research, that the president would take this opportunity to take a really big deep breath and reconsider,'' said Kathy Lewis, president of the Christopher Reeve Paralysis Foundation, named for the late actor who was an outspoken advocate for the science.
In a sense, the issue has come full circle for Mr. Bush. The president devoted his first prime-time television address to the issue, becoming the first president to open the door to federal financing for the science.
Under the policy, which Mr. Bush announced on Aug. 9, 2001, the federal government pays for studies on stem cell colonies, or lines, created before that date, so that the government does not encourage the destruction of additional embryos. Mr. Bush said Wednesday that his administration had made more than $90 million available for such work.
The bill Mr. Bush vetoed would have allowed taxpayer-financed research on lines derived from embryos slated for destruction by fertility clinics. Mr. Bush also signed a ''fetal farming'' measure, barring trafficking in embryos and fetuses with the intent of harvesting body parts.
''These boys and girls are not spare parts,'' the president said in a speech that was interrupted repeatedly by hoots of applause, and twice by standing ovations. ''They remind us of what is lost when embryos are destroyed in the name of research.’’
In one respect, the veto plays to Mr. Bush’s personal strengths, reinforcing the perception that he is someone who makes up his mind and sticks to it, ignoring the polls. But Democrats are determined to make the veto a central theme of their fall election campaigns, hooking it in with another hugely divisive medical issue -- the Terri Schiavo right-to-die case -- to argue that Republicans are beholden to the religious right.
Within hours of the veto, the Senate Democratic leader, Harry Reid of Nevada, sent out a fund-raising letter asserting that Mr. Bush had decided that curing diseases ''was not as important as catering to his right-wing base.'' Representative Edward J. Markey, Democrat of Massachusetts, put it this way: ''This will be remembered as a Luddite moment in American history.''
Even Republicans concede the president’s action could hurt their candidates, particularly moderates like Representative Christopher Shays of Connecticut, who face tough re-election contests.
''It paints us in a corner as more and more single issue, and more and more unreasonable,'' said Ed Rollins, a Republican strategist. ''This is the line that the president certainly doesn’t want Republicans to cross, but I think an awful lot of Republicans say this goes across common sense, this research has the potential of saving my father, my mother, or a friend, or curing cancer.''