But Bush's requirement that federally funded research could come only from existing stem cell lines is, today, too restrictive.

Originally it was widely believed that there were 78 stem cell lines, said Frist. But time has shown that only 22 lines are "eligible" for research.

But beyond that, new science holds all kinds of alternatives, like actually taking adult stem cells and moving them backwards to become embryonic stem cells. Scientists hope that someday embryonic stem cells — which can grow into any other kind of specialized cell — can be used to treat and/or cure diseases.

Such scientific breakthroughs may only come, or may come faster, with federal research, Frist added.

"While human embryonic stem cell research is still at a very early stage, the limitations put in place in 2001 (by Bush) will, over time, slow our ability to bring potential new treatments for certain diseases," Frist said.

Added Hatch, "Stem cell research gives hope to millions of people who have none. More than 100 million Americans suffer from heart disease, cancer, diabetes, Alzheimer's, Parkinson's, multiple sclerosis and so many other life-threatening and life-debilitating diseases" that may well be helped by stem cell research.

In challenging Hatch, Urquhart said he is strongly pro-life, and no real pro-life public official could support changing Bush's stem cell research restrictions to allow the death of an otherwise viable human life.

And Friday, national pro-life leaders criticized Frist.

"Sen. Frist cannot have it both ways. 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," said the Rev. Patrick J. Mahoney, director of the Christian Defense Coalition, who Friday afternoon joined in a anti-Frist press conference outside of the senator's Washington, D.C., offices.

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Contributing: Associated Press.

E-mail: bbjr@desnews.com