Senate Majority Leader Bill Frist announced yesterday that he now supports legislation to lift President Bush's restrictions on federally funded embryonic stem cell research, a shift that infuriated religious conservatives and turned a spotlight back on the White House.

Frist, a physician, said during a lengthy Senate floor speech that he no longer thinks Bush's policy sufficiently enables scientists to pursue the "truly magnificent, truly remarkable properties" of stem cells taken from days-old human embryos.

In the face of a presidential veto threat, the Tennessee Republican with White House aspirations of his own said he will support House-passed legislation to repeal the Bush restrictions and allow research on stem cells donated by couples who have completed in vitro fertilization and no longer need their remaining frozen embryos. He hopes to schedule a Senate vote in September, though he acknowledged widespread disagreement on how to proceed with a debate that could involve up to eight competing bills.

"While human embryonic stem cell research is still at a very early stage, the limitations put in place in 2001 will, over time, slow our ability to bring potential new treatments for certain diseases," Frist said, as other senators flocked to the chamber to hear his speech.

Former first lady Nancy Reagan, some colleagues and Tennessee voters active in the patient advocacy movement hailed Frist for what some called a "courageous" and "principled" stand. At the same time, conservative activists accused him of flip-flopping and betraying his antiabortion beliefs.

Both sides said Frist's announcement is likely to win over some undecided lawmakers and might prompt Bush to reconsider his position.

Sen. Orrin G. Hatch (R-Utah), who has been among the most active supporters of stem cell research, said a veto is no longer a certainty.

"I personally think this ought to convince the president this research should go forward," he said in an interview. As Bush reviews the issue, Hatch said, "he will, like many others, say, 'What's wrong with helping the living?' "

But White House spokesman Scott McClellan said Bush stood by his threat.

"The president's made his position clear," McClellan said. "There is a principle involved here from the president's standpoint when it comes to issues of life."

Scientists, patient groups and a wide majority of Americans embrace the research because of the potential the cells hold for morphing into any type of cell or tissue in the human body. Early work suggests that the regrown cells could enable patients with cancer, Parkinson's disease, diabetes and spinal cord injuries to be treated with their own natural materials.

Because embryonic stem cells can "renew and replicate themselves infinitely," they are "uniquely necessary for potentially treating [other] diseases," Frist said.

Opponents, including religious leaders and several prominent conservative lobbying groups, object to the science because it involves the destruction of embryos.

Sen. Sam Brownback (R-Kan.), expressing "disappointment" with Frist's new position, said lawmakers were ignoring the fundamental question of "whether a young, human embryo is a life or a piece of property." He is pushing several alternative bills that would invest in research that does not require destruction of an embryo.

As a transplant surgeon, Frist has played an influential role in the emotional debate. In July 2001, he outlined an ambitious research agenda on stem cells derived from embryos and adults. But when Bush announced the following month that he would restrict federal money to the embryonic stem cell lines that existed at that time -- estimated at 78 -- Frist acquiesced, and he had maintained until a month ago that Bush's approach would suffice.

Yesterday, in a brief interview, he said he changed his position after learning recently that the two dozen or so embryonic stem cell lines actually available under the Bush policy appear to have weakened as they replicate many times over.

"I knew people said there were only 22" cell lines, he said. "I wasn't aware the stability of the lines was as marginal as they say."

A dozen religious leaders and conservative advocacy groups, in blistering language, attacked Frist for abandoning the Bush policy.

"It certainly gives one pause in trusting his commitment to the sanctity of life," said Lanier Swann, government relations director of Concerned Women for America.

The Christian Defense Coalition said Frist should not expect its support in the 2008 Republican presidential primaries, while the Catholic bishops of Pennsylvania took the opportunity to issue a pointed reminder that the church views embryonic stem cell research as "morally unacceptable."

Catholic League President William Donohue called Frist "Dr. Duplicity." An editorial in the conservative Weekly Standard magazine said: "The incoherence of Frist's position is staggering," while the National Pro-Life Action Center lambasted what it called Frist's "sell-out."

Aides said Frist informed the president of his decision in a phone call Thursday evening, and McClellan said Bush told the majority leader to "vote your conscience."

"The majority leader has given cover to the entire Senate; he's given cover to the entire House of Representatives," said Sen. Arlen Specter (R-Pa.), who characterized Frist's action as an "earthquake." Congressional sources identified Sens. Pat Roberts (R-Kan.) and Kay Bailey Hutchison (R-Tex.) as two lawmakers likely to be swayed to support the House bill.

Some played down the significance of Frist's move.

"Bill Frist is actually being honest about what he believes and who he is. But he is missing the key difference between 2001 and 2005," said Jay Lefkowitz, one of the key architects of the Bush policy. "If the president's position made sense four years ago, which Senator Frist said it did, it makes even more sense today because literally billions of dollars are now available for research. At this point, the opposition to the president's policy is just political; it's really an issue about the allocation of federal health and science dollars."

Sen. Bill Frist favors lifting funding limits.