Promising but still unproven new approaches to creating human embryonic stem cells have suddenly jeopardized what once appeared to be certain Senate passage of a bill to loosen President Bush's four-year-old restrictions on human embryo research.

The techniques are enticing to many conservative activists and scientists because they could yield medically valuable human embryonic stem cells without the creation or destruction of embryos.

Embryonic stem cells are coveted because they have the capacity to become virtually every kind of body tissue and perhaps repair ailing organs, but they are controversial because days-old human embryos must be destroyed to retrieve them.

"The new science that may involve embryo research but not require destruction of an embryo is tremendously exciting," Senate Majority Leader Bill Frist (R-Tenn.) said recently. "It would get you outside of the boundaries of the ethical constraints."

But because the value of these new scientific methods remains speculative, they have complicated the political calculus in the highly partisan Senate, which could take up the issue as early as next week.

Proponents of embryonic stem cell research are divided over how strongly to promote the new work because of fears it will undermine efforts to expand federal funding of conventional embryo research, which they say has better odds of success.

But some opponents of embryo research are uncomfortable with the emerging alternatives, too. That is because they involve cells that closely resemble human embryos, raising novel questions about what, exactly, is a human life.

The science poses a strategic dilemma for both groups: Should they support newly circulating legislation that would fund the new methods or try to defeat what some decry as a Trojan horse?

"This is something that could be very valuable if it works, no doubt about it," Stanford University stem cell researcher Irving Weissman said of the new work. "But don't tell me we should stop doing [embryo] research until we find out, because people's lives are at stake."

In May, the House easily passed bipartisan legislation allowing federally funded scientists to study stem cells derived from some of the thousands of human embryos destined for disposal at fertility clinics -- a significant expansion of the Bush policy. Until this week, Sens. Arlen Specter (R-Pa.) and Orrin G. Hatch (R-Utah) expressed confidence that they had more than enough votes to pass the same bill in the Senate, despite threats of a presidential veto.

Last week, however, opponents began circulating a competing bill that shifts attention toward the more distant but ethically more palatable new procedures. The bill would fund animal studies of new ways to get embryonic stem cells. The measure, sponsored by Rep. Roscoe G. Bartlett (R-Md.), was written with assistance from the White House, said Lisa Lyons Wright, a Bartlett spokeswoman.

The administration is eager for Bush to sign legislation supportive of at least some types of stem cell research, according to several lobbyists close to the congressional negotiations. Signing such a bill could take some of the sting out of a veto that is sure to infuriate patient groups and could rile a majority of Americans, who tell pollsters they back expanded funding of embryonic stem cell research.

During the Fourth of July recess, many Senate Republicans struggled with the question of whether the new legislation should be brought to the floor as a substitute for the House-passed bill or as a competing bill -- and if both were to come up, then how to vote on each. At least a handful of senators have hinted in recent days that they may transfer their vote to the new bill, Hill sources said -- among them Hatch, Johnny Isakson (R-Ga.) and Kay Bailey Hutchison (R-Tex.).

The issue will get its first formal airings at a Senate subcommittee hearing Tuesday and at a Hill media event on Wednesday at which pro-research celebrities Michael J. Fox and Dana Reeve, widow of "Superman" star Christopher Reeve, will call for an immediate loosening of Bush's policy.

Some supporters of the research say they would be happy if both bills passed. But for some of the more ardent advocates of an immediate expansion of the Bush policy, Bartlett's alternative legislation is a diversion.

"Don't stop embryonic stem cell research now, hoping there will be some other way to do it in the future," Sen. Tom Harkin (D-Iowa) said in an interview. "These alternative methods of deriving stem cells -- we don't know whether they'll work. The one thing we do know how to do is derive embryonic stem cells."

The new techniques fall into two major categories. In one, a single cell is removed from a days-old embryo created for fertility purposes and coaxed to become a self-replicating colony of stem cells, leaving the remainder of the embryo to develop normally. The technique shows great promise, according to researchers at Advanced Cell Technology Inc. in Worcester, Mass., who pioneered it. But critics have raised the possibility that individual cells removed from such young embryos may have the biological potential to become embryos themselves, which would mean their destruction or cultivation as colonies could still raise ethical issues.

Bush's Council on Bioethics also expressed concerns recently that such a technique may subtly harm an embryo, even if it does not kill it.

"You may get a human being, but you may not get the same human being," said William B. Hurlbut, a Stanford professor and a council member. "You might find that late in life, there are some strange differences between those people and others."

Hurlbut is the leading proponent of a different approach, which he calls altered nuclear transfer, or ANT. It involves the creation of an embryo -- or what Hurlbut says is something akin to an embryo -- that lacks a gene necessary for the development of a placenta. Because a placenta is required for an embryo to implant in a woman's womb, the altered embryo would be genetically incapable of becoming a fetus or a baby. For many, that would obviate ethical concerns about destroying it to get its stem cells.

Researchers have tried the technique in mice with some success, but its usefulness as a source of human stem cells remains hypothetical. Some, such as Weissman, think the difficulties inherent in making such a system work are being overlooked by Hurlbut, who is a physician but not a research scientist.

"I've been telling Bill, 'Why don't you go work in a lab this summer? Why not see how easy or hard it really is?' " said Weissman. He said he has no problem with the funding of such research as long as it does not interfere with increased funding for existing programs of embryo research.

Practical or not, ANT has gained a quickly widening circle of support. The Roman Catholic archbishop of San Francisco, William J. Levada, has written a letter to Bush assuring the president of his support.

But other conservative leaders have mixed views on whether it makes sense to pursue the new alternative therapies or to focus single-mindedly on defeating any expansion of the current policy.

"I have significant concerns about all the alternatives," said David Prentice, senior fellow for life sciences at the Family Research Council, which he said does not yet have a formal position on the science.

Jessica Echard, executive director of the Eagle Forum, the public policy organization founded by Phyllis Schlafly, said her group opposes "middle ground" legislation that pursues alternative methods for producing embryonic stem cells.

"Most scientists will say it's never enough," she said. "We will be giving ground to more and more unethical research."