Breaking with President Bush, Senate Majority Leader Bill Frist (search) on Friday threw his support behind legislation to expand federal funding for human embryonic stem cell research so long as it stays "within ethical bounds."

"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 of support for the House-passed legislation immediately dented his support among Christian conservatives but won praise from Democrats, as well as from former first lady Nancy Reagan, whose husband, the late former President Ronald Reagan, had Alzheimer's disease for years before his death.

"Embryonic stem cell research (search) has the potential to alleviate so much suffering," Nancy Reagan said. "Surely, by working together we can harness its life-giving potential."

Click to the video box to the right for a report by FOX News' Major Garrett.

Bush supports only limited federal funding for embryonic stem cell research and has argued that life should not be created for the purpose of being destroyed during research. Private funding on these cells, however, is not restricted by federal law.

Opponents of expanding research on embryos say there is not enough medical proof to show that embryonic cells may yield more medical miracles than adult stem cells. But proponents of expanding embryonic stem cell funding say the more research done on the widest variety of cells, the better the chances of finding potential cures for crippling diseases.

At the White House, press secretary Scott McClellan said Frist had given Bush advance notice of his announcement. Frist also notified Mrs. Reagan and House Speaker Dennis Hastert, R-Ill., of his pending announcement in telephone calls late Thursday, one Republican congressional source told The Associated Press.

"The president and Senator Frist had a good discussion last night. They were discussing legislative priorities when Senator Frist mentioned his intention regarding stem cells," McClellan said. "The president told the leader, 'You have to go with your conscience.'"

McClellan said Bush still 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.

"I made my position very clear on embryonic stem cells (search). I'm a strong supporter of adult stem cell research, of course," the president said on May 20. "But I made it very clear to the Congress that the use of federal money, taxpayers' money to promote science which destroys life in order to save life is — I'm against that. And therefore, if the bill does that, I will veto it."

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.

"Senator 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 (search), 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 (search), 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."

But his 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."

GOP: Frist Advocating 'Bad Policy'

House Republicans, however, said they were "profoundly disappointed" in Frist's decision, saying federal funding of human embryonic stem cell research is fiscally irresponsible because it's unproven technology.

"Senator Frist is a good man; he's simply advocating a bad policy," said House Majority Leader Tom DeLay of Texas.

Rep. Phil Gingrey, R-Ga., rejected the praise showered upon Frist for bucking his party and the White House. "I think the courageous speech would have been to continue to stand strongly in defense of the sanctity of life," Gingrey, a physician, told reporters. "We will fight him on this to the very end."

"House conservatives are profoundly disappointed at Senator Frist's decision to abandon this cause," said Rep. Mike Pence, R-Ind.

On the Senate side, Sen. Rick Santorum, R-Pa., said he was "disappointed" at Frist's decision.

"Nevertheless, I will continue to work hard to lay out ethical alternatives to the destruction of human life for experimentation," Santorum said. "I don't think society should sacrifice the least among us for the purpose of science."

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.

StemPAC, a group supporting such research, this week launched a television ad in New Hampshire criticizing Frist for not scheduling a vote on the issue. Frist added on Friday that he expected debate and a vote when the Senate returns from vacation in the fall.

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.

The Rev. Carlton Veazey, president of the Religious Coalition for Reproductive Choice, sad Frist's announcement "gives hope that the leaders of this country will stop playing politics with people's health."

"Some members of Congress and interest groups have stalled this legislation by unconscionably placing their ideology above the lives of Americans with incurable diseases. It's time to end the ideological stranglehold and pass legislation to fund this life-saving research," Veazey said.

But groups such as American Values, led by former presidential candidate Gary L. Bauer, said Frist had essentially "abandoned" the president on the issue.

"The senator seems to have bought into the notion that some life can be sacrificed without its consent to benefit other life, and that is a slippery slope with deadly consequences, as history has shown repeatedly," Bauer said.

The National Pro-Life Action Center said the crux of the issue is not whether studying more embryonic cells will yield more medical benefits but whether "it is ever morally licit to take the life of one innocent human being to potentially benefit another. The answer to that is unequivocally 'No.' It is never morally acceptable."

The Associated Press contributed to this report.