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President Bush continued to defend his stated policy on federal funding for embryonic stem cell research Saturday, even as opponents on both sides of the issue continued to criticize his decision to limit but not ban government money for the studies.

"As we go forward, I hope we'll always be guided by both intellect and heart," the president said in his weekly radio address, "by both our capabilities and our conscience."

As he did in his nationally televised address Thursday night, Bush made his position clear: "I have concluded that we should allow federal funds to be used for research on these existing stem cell lines where the life-and-death decision has already been made."

Stem cells can be grown into almost any kind of human tissue, and medical researchers are hopeful that they can be used to treat currently incurable conditions, such as spinal-cord injuries, Parkinson's disease and Alzheimer's disease. While each individual has some stem cells, the brand-new stem cells in a human embryo a few days old are considered to have the most potential for medical breakthroughs.

The embryos that have been used to provide stem cells until now have been the byproducts of in-vitro fertility treatments, where several embryos are created for each successful pregnancy, the "leftovers" commonly being discarded.

Bush will not allow federal funding for research on stem cells that have been harvested from newly created embryos, but rather on existing self-sustaining colonies of stem cells that originated in embryos long ago discarded.

"This allows us to explore the promise and potential of stem cell research without crossing a fundamental moral line, by providing taxpayer funding that would sanction or encourage further destruction of human embryos that have at least the potential for life," Bush said.

The president on Thursday night put the number of existing lines worldwide at 60, but many medical experts either disputed that figure or said that even if it were correct, it would not provide the necessary genetic diversity to permit successful research.

Limiting the cell lines "would be a very poor investment and a very cruel investment" if science ended up with stem cells that would be incompatible with many patients, said Dr. Harold Varmus, a Nobel laureate who headed the National Institutes of Health under former President Clinton.

Some current NIH researchers say that scientific journals and literature list less than a dozen existing stem cell lines and that some of these lines are controlled by private companies that would not seek federal funding.

In addition, they say, some stem cell lines that may exist around the world would not be eligible at the NIH because of an "informed consent" provision adopted during the Clinton administration. "Informed consent" requires that U.S. scientists only conduct research on embryonic stem cell lines derived from couples that have given written consent, and many researchers around the world do not face this consent provision.

Roman Catholic leaders condemned the president's decision as "morally unacceptable." Conservative Protestants said they were disappointed but encouraged by Bush's thoughtful approach.

"The fact that he is not putting federal funds in the support of killing additional babies is a very critical line not crossed," said Richard Land, president of the Southern Baptists' Ethics and Religious Liberty Commission.

On the political side of the stem cell equation, some said Bush may have done what he needed to do on Capitol Hill to support research while holding off those who want Congress to forbid the use of government money for embryonic research of any kind.

"The president probably bought himself some time," said Thomas Mann, a congressional scholar at the Brookings Institution. "Pressure will build again, but it will take some time."

Both sides expect debate over the issue to resume in Congress when lawmakers return next month.

Research advocates can count on a majority in the Senate and nearly half the House, and some have vowed to push for broader funding.

"Restrictions on this lifesaving research will slow the development of the new cures that are so urgently needed by millions of patients across America," said Sen. Edward Kennedy, D-Mass.

"We just have to watch this play out," said Sen. Orrin Hatch, R-Utah, a conservative who opposes abortion but has supported stem cell research. He told reporters Friday he would like to see more stem cell lines available but that Congress should hold back for now. "Let's give it a chance," he said.

The Associated Press contributed to this report