"It's not just a matter of faith, it's a matter of science," Frist, R-Tenn., said on the floor of the Senate.
Frist's announcement immediately dented his support among Christian conservatives but won lavish praise from former first lady Nancy Reagan, who said it "has the potential to alleviate so much suffering." Her husband, the late former President Ronald Reagan, had Alzheimer's disease.
At the White House, press secretary Scott McClellan said Frist had given Bush advance notice of his announcement. "The president said, `You've got to vote your conscious,'" McClellan said.
"The president's made his position clear," the spokesman said when asked if Bush stands by his threat to veto a pending bill that would liberalize federal support for stem cell research. "There is a principle involved here from the president's standpoint when it comes to issues of life," McClellan said.
Bush and Frist appeared together at the White House shortly after Frist's speech as the president signed a bill that allows health care professionals to report information on medical errors without fearing that it will be used against them in lawsuits.
Bush introduced him as "Doctor Bill Frist" and afterward, Bush shook Frist's hand and said something that made the majority leader laugh. As Bush was leaving the room, he summoned Frist to join him.
The Christian Defense Coalition lambasted Frist's change of position.
"Sen. Frist should not expect support and endorsement from the pro-life community if he votes for embryonic research funding," it said.
"Senator Frist cannot have it both ways. He cannot be pro-life and pro-embryonic stem cell funding," said Rev. Patrick J. Mahoney, director of the group. "Nor can he turn around and expect widespread endorsement from the pro-life community if he should decide to run for president in 2008."
A heart-lung transplant surgeon who opposes abortion, Frist said loosening Bush's strict limitations on stem cell research would lead to scientific advances and "bridge the moral and ethical differences" that have made the issue politically charged.
"While human embryonic stem cell research is still at a very early stage, the limitation put into place in 2001 will, over time, slow our ability to bring potential new treatments for certain diseases," the Tennessee lawmaker said in his speech.
"Therefore, I believe the president's policy should be modified. We should expand federal funding ... and current guidelines governing stem cell research, carefully and thoughtfully, staying within ethical bounds," he said.
Bush has threatened to veto legislation for expanded financial support for stem cell research. A bill to finance more stem cell research has passed the House, but has been stalled in the Senate. Frist's support could push it closer to passage and set up a confrontation with Bush.
Frist's announcement will put pressure on the White House, predicted Sen. Arlen Specter, R-Pa., a cancer patient and the bill's sponsor.
"I know that the president will listen to what Sen. Frist has had to say," Specter said. "I'm not saying he is going to agree with it but ... I think may bring us all together on this issue."
It also could impact Frist's own political future. A likely presidential candidate in 2008, Frist has been courting religious conservatives who helped make Bush a twice-elected president and generally consider embryonic stem cell research a moral equivalent to abortion. But the announcement, coming just a month after Frist said he did not support expanded financing "at this juncture," could help him with centrist voters.
With those political realities in mind, Frist argued that his positions on stem cell research and abortion were not inconsistent. He said the decision was about policy, not politics.
Frist's decision brought quick praise from leading Democrats.
"It is a decision that will bring hope to millions of Americans," said Senate Democratic leader Harry Reid of Nevada. "I know there's still a long ways to go with the legislation, but a large step has been taken by the majority leader today ... and I admire the majority leader for doing it."
Said Sen. Edward M. Kennedy, D-Mass.: "As a physician, Sen. Frist has a moral calling to save lives and alleviate suffering. He honors his Hippocratic Oath today by recognizing the unique healing power of embryonic stem cells."