After months of deliberations, President Bush announced approval Thursday night for federal funding for limited medical research on stem cells extracted from human embryos.
In the first prime-time speech of his presidency, Bush said the issue was "one of the most profound of our time," adding he had come down in favor of funding because of the potential for new cures for chronic diseases.
Bush, seeking a middle ground on the issue, said he would limit federal funding for existing lines of embryonic stem cells. That would restrict the research to embryos that have already been destroyed.
But Bush, an opponent of abortion, said he would permit federal funding only for embryonic stem cells that are already in existence. He said it was "important that we pay attention to the moral concerns of the new frontier."
"I have made this decision with great care and I pray it is the right one," he said.
Bush made his speech from a house on his Texas ranch, far from the White House, where opponents of stem cell research marched earlier in the day.
The decision was sure to fuel an emotional debate, and exposed Bush to sharp criticism from conservatives unalterably opposed to any type of embryonic stem cell research.
In his speech, Bush said he had read and consulted widely before reaching his decision. In a politically poignant reference, he said he had heard from former first lady Nancy Reagan, an opponent of abortion who supports research. Her husband was a staunch opponent of abortion who suffers from Alzheimer's disease, one of the illnesses that scientists hope to conquer through stem cell research.
"As I thought through this issue I kept returning to two fundamental questions," Bush said.
"First, are these frozen embryos human life and therefore something precious to be protected? And second, if they're going to be destroyed anyway, shouldn't they be used for a greater good, for research that has the potential to save and improve other lives?
In reaching his decision, Bush sorted through conflicting advice from ethicists, anti-abortionists, advocates for the ill, researchers and Pope John Paul II.
At issue was whether the government should support research on stem cells removed from embryos that are left over from fertility treatments. Supporters of such research see great potential for medical treatments. Opponents insist it is wrong to use human embryos for research. Bush, as a candidate, opposed federal funding for research that destroys human embryos.
Stem cells are created by removing an inner cell mass from a 5- to 7-day-old embryo. The procedure kills the embryo. When properly nurtured, the cells are able to replicate, or divide, virtually forever, creating what is called a stem cell line.
There are about a dozen embryonic stem cell lines now in existence, experts say, but some of them could not be used in federal research because they were not produced within the rules set by the National Institutes of Health.
Stem cells are capable of developing into any of the body's organs but not into a complete individual. These cells form inside an embryo a few days after fertilization.
By properly nurturing embryonic stem cells, experts believe they can grow new cells to restore ailing organs in chronically ill patients. For instance, new insulin-producing cells could be grown, perhaps to cure diabetes.
Congress has banned government money for stem cell research that destroys embryos. But the Clinton administration ruled that such research could receive federal funding as long as private money financed the part of the process that actually destroyed the embryo -- the extraction of the stem cells. Bush delayed such funding while he reviewed the policy.
"This is a serious, difficult issue that the president has approached in a deliberate and thoughtful manner over the course of the last several weeks and since the beginning of this administration," White House spokesman Scott McClellan said.
"This is a decision that will have far-reaching implications for our nation 20 to 30 years from now and beyond," he said.
Bush made his decision Wednesday after a meeting with top aides on his ranch in central Texas. Aides maintained the secrecy that characterized the process, giving 14 hours notice of his nationally televised address without revealing the decision to anyone but a tiny circle of advisers.
Thompson, who has been pushing the president to let the research advance, said: "I am fairly comfortable with the decision that the president is going to make and I'm very confident that the American people will be as well." Thompson spoke on ABC's "Good Morning America" before he was advised of Bush's decision.
McClellan said Bush made his decision "based on what he believes is in the best interest of the American people," rejecting any suggestion that politics played a role.
But the decision will have broad political ramifications, and Bush's top political adviser, Karl Rove, was deeply involved.
The Republican president has courted Roman Catholic voters, a key electoral bloc. Several church leaders -- including Pope John Paul II -- urged the president to bar funding for embryonic stem cell research.
A decision to allow it could alienate some Catholics. But recent polls show a majority of both Catholics and Protestants support it. Practicing Catholics are more divided on the topic.
Six in 10 Americans say they support stem cell research, and a similar proportion say they support federal funding. About half of Republicans support it, while four in 10 oppose it.
Among those Bush consulted were Catholic leaders, National Institutes of Health scientists and representatives of the National Right To Life Committee and the Juvenile Diabetes Foundation.
The last expert that Bush heard from on the issue was a pro-research bioethicist who spoke with the president in the Oval Office last Thursday afternoon, aides said.
White House officials coordinated Thursday morning with the Republican National Committee on its weekly conference call with some 30 conservative activists. Republicans described those on the call, including the Christian Coalition and other groups opposed to embryonic cell research, as eager to know what the decision was -- only to be told there would be no details until the president's address.
Bush put the finishing touches on his speech Thursday, went jogging and fished.
The Associated Press contributed to this report