WASHINGTON, July 29 - In a break with President Bush, the Senate Republican leader, Bill Frist, has decided to support a bill to expand federal financing for embryonic stem cell research, a move that could push it closer to passage and force a confrontation with the White House, which is threatening to veto the measure.

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"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," Mr. Frist said. "Therefore, I believe the president's policy should be modified."

His speech received the approval of Democrats as well as Republicans.

"I admire the majority leader for doing this," Senator Harry Reid, the minority leader and Democrat of Nevada, said immediately after the speech. He and Senator Dick Durbin, Democrat of Illinois, said Mr. Frist's stance would give hope to people everywhere.

Senator Arlen Specter, Republican of Pennsylvania, contending they were discussing "the difference between life and death," said of Mr. Frist, "I believe the speech that he has just made on the Senate floor is the most important speech made this year, and perhaps the most important speech made in years."

He added: "This is a speech that will reverberate around the world, including at the White House."

Scott McClellan, Mr. Bush's chief spokesman, said Mr. Frist had told Mr. Bush in advance notice of his planned announcement. "The president said, "You've got to vote your conscience," Mr. McClellan said, according to The Associated Press.

"The president's made his position clear," Mr. McClellan said when asked if Mr. Bush would veto a pending bill that would liberalize federal support for stem cell research, The A.P. reported. "There is a principle involved here from the president's standpoint when it comes to issues of life."

Mr. Frist's move will undoubtedly change the political landscape in the debate over embryonic stem cell research, one of the thorniest moral issues to come before Congress. The chief House sponsor of the bill, Representative Michael N. Castle, Republican of Delaware, said, "His support is of huge significance."

The stem cell bill has passed the House but is stalled in the Senate, where competing measures are also under consideration. Because Mr. Frist's colleagues look to him for advice on medical matters, his support for the bill could break the Senate logjam. It could also give undecided Republicans political license to back the legislation, which is already close to having the votes it needs to pass the Senate.

The move could also have implications for Mr. Frist's political future. The senator is widely considered a potential candidate for the presidency in 2008, and supporting an expansion of the policy will put him at odds not only with the White House but also with Christian conservatives, whose support he will need in the race for the Republican nomination. But the decision could also help him win support among centrists.

"I am pro-life," Mr. Frist said in the speech, arguing that he could reconcile his support for the science with his own Christian faith. "I believe human life begins at conception."

But at the same time, he said, "I also believe that embryonic stem cell research should be encouraged and supported."

Tony Perkins, the president of the Family Research Council, a conservative Christian group, said today in a statement that Senator Frist's decision was "very disappointing but not a surprise," given the senator's previous testimonies advocating stem cell research.

"As a heart surgeon who knows that adult stem cells are already making huge progress in treating heart disease in humans, it is unfortunate that Sen. Frist would capitulate to the biotech industry," Mr. Perkins said. "Thankfully, the White House has forcefully promised to hold the ethical line and veto any legislation that would expand the president's current policy."

Rev. Patrick J. Mahoney, director of the Christian Defense Coalition, also objected to Mr. Frist's decision and alluded to its political impact. "Senator Frist cannot have it both ways," he said, according to The A.P. "He cannot be pro-life and pro-embryonic stem cell funding. Nor can he turn around and expect widespread endorsement from the pro-life community if he should decide to run for president in 2008."

Backers of the research were elated. "This is critically important," said Larry Soler, a lobbyist for the Juvenile Diabetes Research Foundation. "The Senate majority leader, who is also a physician, is confirming the real potential of embryonic stem cell research and the need to expand the policy."

Mr. Frist, who was instrumental in persuading President Bush to open the door to the research four years ago, has been under pressure from all sides of the stem cell debate. Some of his fellow Senate Republicans, including Orrin G. Hatch of Utah and Mr. Specter, who is the lead Senate sponsor of the House bill, have been pressing him to bring up the measure for consideration.

"I know how he has wrestled with this issue and how conscientious he is in his judgment," Mr. Specter said today. "His comments will reverberate far and wide."

But with President Bush vowing to veto it - it would be his first veto - other Republicans have been pushing alternatives that could peel support away from the House bill.

Last week Mr. Castle accused the White House and Mr. Frist of "doing everything in their power to deflect votes away from" the bill. On Thursday night, Mr. Castle said he had written a letter to Mr. Frist just that morning urging him to support the measure. "His support of this makes it the dominant bill," he said.

Despite Mr. Frist's speech, a vote on the bill is not likely to occur before September because the Congress is scheduled to adjourn this weekend for the August recess.

With proponents of the various alternatives unable to agree on when and how to bring them up for consideration, Mr. Frist says he will continue to work to bring up all the bills, so that senators can have a "serious and thoughtful debate."

Human embryonic stem cells are considered by scientists to be the building blocks of a new field of regenerative medicine. The cells, extracted from human embryos, have the potential to grow into any type of tissue in the body, and advocates for patients believe they hold the potential for treatments and cures for a range of diseases, from juvenile diabetes to Alzheimer's disease.

"Embryonic stem cells uniquely hold some promise for specific cures that adult stem cells just cannot provide," Mr. Frist said.

But the cells cannot be obtained without destroying human embryos, which opponents of the research say is tantamount to murder. "An embryo is nascent human life," Mr. Frist said in his speech, adding: "This position is consistent with my faith. But, to me, it isn't just a matter of faith. It's a fact of science."

On Aug. 9, 2001, in the first prime-time speech of his presidency, Mr. Bush struck a compromise: he said the government would pay only for research on stem cell colonies, or lines, created by that date, so that the work would involve only those embryos "where the life or death decision has already been made."

The House-passed bill would expand that policy by allowing research on stem cell lines extracted from frozen embryos, left over from fertility treatments, that would otherwise be discarded. Mr. Castle has said he believes the bill meets the president's guidelines because the couples creating the embryos have made the decision to destroy them.

In his speech, Mr. Frist seemed to adopt that line of reasoning, harking back to a set of principles he articulated in July 2001, before the president made his announcement, in which he proposed restricting the number of stem cell lines without a specific cutoff date. At the time, he said the government should pay for research only on those embryos "that would otherwise be discarded" and today he similarly supported studying only those "destined, with 100 percent certainty, to be destroyed."

Moreover, he said, "Such funding should be provided only within a comprehensive system of federal oversight."

After Mr. Bush made his 2001 announcement, it was believed that as many as 78 lines would be eligible for federal money. "That has proven not to be the case," Mr. Frist said. "Today, only 22 lines are eligible."

But, Mr. Frist says the Castle bill has shortcomings. He says it "lacks a strong ethical and scientific oversight mechanism," does not prohibit financial incentives between fertility clinics and patients, and does not specify whether the patients or the clinic staff have a say over whether embryos are discarded. He also says the bill "would constrain the ability of policy makers to make adjustments in the future."

Mr. Frist also says he supports some of the alternative measures, including bills that would promote research on so-called adult stem cells and research into unproven methods of extracting stem cells without destroying human embryos.

"Cure today may be just a theory, a hope, a dream," he said in conclusion today. "But the promise is powerful enough that I believe this research deserves our increased energy and focus. Embryonic stem cell research must be supported. It's time for a modified policy - the right policy for this moment in time."