Bush said there are about 60 existing stem cell lines in various research facilities -- cell lines that have already been derived from human embryos.

"I have made this decision with great care, and I pray that it is the right one," Bush said in a nationally televised address from his ranch in Crawford, Texas, where he is on a monthlong working vacation.

Scientists and advocacy groups view embryonic stem cell research as perhaps the best hope for finding cures for debilitating diseases like Alzheimer's and Parkinson's.

Other groups, such as anti-abortion activists, consider stem cell research the taking of a human life because embryos must be destroyed to harvest the stem cells.

Some of Bush's closest advisers - including Health and Human Services Secretary Tommy Thompson -- had urged him to allow broader funding of the controversial science.

One compromise that Bush reportedly had been considering would have allowed the funding of research using stem cells from excess embryos at fertility clinics that had been destined to be destroyed.

Bush opted not to go that far. He said he would allow funding for research using existing stem cell lines only, "where the decision on life and death has already been made."

Conservative groups had called upon Bush to stick to campaign promises to reject any federal funding for embryonic stem cell research.

Bush said research using embryonic stem cells involved "great promise, and great peril."

"We must proceed with great care," Bush said.

Grappling with the issue
Bush, sources said, previously had decided to flatly oppose federal funding of research that involved or used embryos gathered solely for research purposes, or embryos created through cloning human cells.

The issue Bush grappled with was whether to stand by his previous statements opposing federal funding for any embryonic stem cell research, or to reverse course and support the position backed by many of his closest advisers, including Thompson, Vice President **** Cheney, Chief of Staff Andy Card and White House counselor Karen Hughes, according to sources.

Among those who recommended against any change in position, these sources said, were strategist Karl Rove, the top White House liaison to conservative Republicans.

During the presidential campaign, Bush said he opposed federal funding of embryonic stem cell research.

And in a May 18, 2001, letter to a group called The Culture of Life Foundation, Bush wrote: "I oppose federal funding for stem cell research that involved destroying living human embryos." Bush went on to say he supported research using stems cells from adult donors.

But the president came under heavy pressure to reconsider and had grappled with the issue for more than two months, holding dozens of meetings with medical and scientific experts, ethicists, religious leaders and others.

He also raised the issue at events on other issues, including a meeting with doctors to discuss the patients' bill of rights and an event that included breast cancer survivors.

White House officials said Bush reached his decision since arriving in Texas for his working vacation and decided Wednesday he wanted to announce it on Thursday in the nationally televised address. These officials said he had made clear he wanted to be the first to disclose it.

Funding could alienate conservatives
Bush's decision could put him at odds with many cultural conservative organizations -- and key GOP leaders in the House. Majority Leader **** Armey and Majority Whip Tom DeLay, for example, have said they would lead an effort to block any Bush attempt to allow federal funding.

House Speaker Dennis Hastert has said he personally opposes embryonic stem cell research, but he has not embraced the tough words of his deputies and was working with the White House to quiet talk of any major public GOP leadership revolt over Bush's decision.

Leading Democrats, on the other hand, have vowed to press for legislative language allowing federal funding regardless of what the president announces.