First lady Laura Bush (search) defended her husband's policy on embryonic stem cell research Monday, calling Democratic rival John Kerry's (search) criticism "ridiculous" and accusing proponents of overstating the potential for medical breakthroughs.

"We don't even know that stem cell research will provide cures for anything — much less that it's very close" to yielding major advances, Mrs. Bush said.

The first lady weighed in on the highly charged political and scientific issue on the third anniversary of Bush's decision to limit federal funding of embryonic stem cell research (search) to only the 78 stem cell lines in existence Aug. 9, 2001.

Religious groups oppose the scientific work in which culling of stem cells kill the embryos, equating that with abortion, and had urged Bush not to be the first president to fund the research — even with limits.

Proponents of the science, including former first lady Nancy Reagan (search) and 58 Democrats and Republicans in the Senate, argue that it could lead to cures to diseases such as diabetes and Alzheimer's. Former President Reagan suffered from the latter for a decade before his death June 5 due to related pneumonia.

Only a fraction of those initial 78 stem cell lines — 21 at last count — are yet available to researchers because of problems with the lines' growth or their ownership. In March, a National Institutes of Health count cast doubt on how many ultimately would be usable.

Proponents and members of the medical community say more than 100 new cell lines have been created worldwide since Bush's decision — some with new techniques that may make them more scientifically useful — and could be studied under more open rules. An exact count isn't possible because private funding means much of the work is done without any public scrutiny.

With polls showing overwhelming support for stem cell research, Kerry has promised to give scientists more freedom. He has used the word ban to describe Bush's actions when what the president has done is limit the research.

"That's so ridiculous," Laura Bush said in an interview with The Associated Press, calmly fielding questions about her husband and his presidential race.

Mrs. Bush added matter-of-factly: "It's one of the myths that start during a campaign."

Kerry spokesman Phil Singer said Bush's restrictions apply to 99.9 percent of potential stem cell lines that could be studied. "If that's not a ban," he said, "we don't know what is."

Unusually combative, the first lady said Kerry was trying to make a political issue out of her husband's policy "without saying what's right. I imagine he knows better."

Like other Bush-Cheney campaign surrogates, Mrs. Bush credited her husband with being the first president to use taxpayer money for the research. That is true, perhaps only because the science is so new. The policy of Democratic President Clinton allowed taxpayer money to be used in the research of any stem cell lines, but he never funded the nascent research. Bush invested $25 million in limited research.

Kerry's running mate, Sen. John Edwards of North Carolina, said Monday marked "a sad anniversary" because the Bush administration "put restrictions in place that dramatically undermine our efforts to find cures for diseases."

Edwards, in a conference call with reporters, said Kerry would reverse Bush's policy, invest $100 million for research and establish ethical guidelines for the science.

In a speech to the Pennsylvania Medical Society, which endorsed her husband, Mrs. Bush said policy-makers must be aware of the "ethical and moral implications" of the research.

"I hope that stem cell research will yield cures," the first lady said. "But I know that embryonic stem cell research is very preliminary right now and the implication that cures for Alzheimer's are around the corner is just not right and it's really not fair to people who are watching a loved one suffer with this disease."

Afterward, the first lady said she was not singling out the Reagan family, several of whom have called for the restrictions to be lifted. Ron Reagan spoke out against Bush's policy at the Democratic National Convention last month, urging delegates to cast a vote for stem cell research in November — a tacit endorsement of Kerry.

"It's not fair" to raise false hopes "because stem cell research is very, very preliminary," she said. Alzheimer's contributed to the death of her father in the 1990s.

Health and Human Services Secretary Tommy Thompson also defended Bush's policy, saying the administration "opened the doors for the first time to federal taxpayer funding for human embryonic stem cell research." Clinton's policy would have paid for research using stem cell lines created at any time.

Embryonic stem cells are master cells that form during the early days after conception and can turn into any tissue in the body. Many scientists hope to one day harness them to grow replacement tissue to treat diabetes, spinal cord injuries and other diseases.

A poll by the University of Pennsylvania National Annenberg Election Survey found that 64 percent of Americans favor federal funding of embryonic stem cell research while 28 percent oppose it. Independent voters, crucial in a close election, also back using taxpayers' dollars while slightly more than half of Republicans support it, according to the survey released Monday.

While Bush's actions forbid federal dollars, it does not stop private funding of stem cell research. In an editorial in the New England Journal of Medicine, Dr. George Q. Daley, a leading stem cell researcher, said that research has struggled without federal funds.