The debate over embryonic stem cell research reached new heights of philosophical complexity yesterday as a Senate subcommittee wrestled with the question of whether human embryos intentionally endowed with fatal genetic flaws would still be too human to justify their mass production for experiments.

The literal bout of soul-searching on Capitol Hill was prompted by opponents of embryo cell research, who hope to undermine support for a bill that would loosen President Bush's four-year-old restrictions on the controversial field.

Central to the newly emerging conservative strategy is an effort to encourage researchers to get the medically promising cells from alternative, albeit unproven, sources instead of from human embryos. Prime among those alternatives are embryos that might not pass muster as "human" because they have been engineered to lack a gene crucial for development into a baby.

The lobbying effort has resulted in the unusual situation of conservative opponents of embryo research, rather than scientists, proposing stem cell experiments that some ethicists say raise profoundly troubling issues.

The approach would be tantamount to "deliberately creating and then destroying an impaired form of human life," Ronald M. Green, director of Dartmouth College's Ethics Institute, told the Senate Appropriations subcommittee on labor, health and human services and education.

If the moral logic of the approach is left unchallenged, Green warned, society could find itself heading down a slippery slope in which scientists feel it is okay to produce babies without brains as sources of organs for transplantation.

Conservatives are not unanimous in their support for the new approach to making stem cells. But supporters say the genetic glitch would be inserted into a single cell before it became an embryo, so the resulting "entity" -- though long-lived enough to generate stem cells -- would face a developmental dead end and would therefore be suitable fodder for research.

"There is no embryo there," said Stanford University professor William Hurlbut, a leading proponent of the approach and a member of President Bush's Council on Bioethics. Hurlbut referred to the engineered balls of cells as "constructs" subjected to "preemptive alteration."

He said that this and a range of related proposals he has been circulating have garnered "wide support of moral philosophers and religious authorities."

But some lawmakers expressed skepticism that the technical end run represented an improvement over methods that would be allowed under the pending Senate bill. That legislation, passed by the House with bipartisan support in May, would allow federal funding of research on cells from excess embryos already slated for destruction at fertility clinics.

"When the option is to throw them away or use them [in research], it seems to me a clear-cut choice," Sen. Arlen Specter (R-Pa.), subcommittee chairman, said testily. He has expressed his vexation over stem cell research limits with increasing passion since his own cancer diagnosis last year.

Former Appropriations Committee chairman Ted Stevens (R-Alaska) said he agreed with Specter "100 percent," and Sen. Tom Harkin (D-Iowa), the subcommittee's minority leader, criticized Hurlbut's approach as monstrous.

"If it's not an embryo, what is this Frankenstein-like thing we're creating?" he asked. "The hypocrisy here is indefensible."

Embryonic stem cells are coveted for their potential to repair damaged organs, but they have stirred controversy because their only known source is human embryos, which are destroyed in the process.

Under the Bush policy, federal funds may be used to study only those stem cells derived by Aug. 9, 2001 -- a restriction that has kept the federal scientific enterprise from studying the more than 100 colonies of stem cells developed since then, many of which appear superior to the older colonies.

Stem cell politics have grown immensely complicated in the past two weeks as opponents of the research realized they were likely to lose in the Senate -- a situation that would force Bush to decide whether to invoke a promised veto.

As of yesterday, it appeared that conservatives were drafting as many as six different stem cell-related bills, including ones that would encourage more research on adult stem cells; fund animal studies of alternative sources of stem cells; and ban human cloning.

With wording still in flux, it was unclear yesterday whether any of those bills would conflict with or preclude passage of the original bill to loosen the Bush rules. If not, senators might be able to vote for all of them and be able to say they support the search for alternative methods while also supporting the one approach that so far has been shown to work -- deriving stem cells from spare fertility clinic embryos.

Daniel Perry, president of the Coalition for the Advancement of Medical Research, which represents patient groups and other pro-stem cell interests, said the alternative approaches to generating stem cells amount to "a catch-all bag of speculation, unpublished science, wishful thinking and philosophical pretzel-bending."

Perry said he would be happy to support a search for alternatives if it did not undermine the primary effort to liberalize the Bush policy.

Robert Lanza, of Advanced Cell Technology of Worcester, Mass., who is developing a different way to get stem cells without harming embryos, agreed that improved access to conventionally derived stem cells should be the priority for now. "That's a no-brainer," he said.

Nick Smith, a spokesman for Senate Majority Leader Bill Frist (R-Tenn.), said Frist has not yet decided which bills he will support or how many would be folded into a unanimous consent agreement now under construction. He said a vote is expected by the end of July.

A coalition of 68 religious leaders of various faiths yesterday sent a letter to Frist urging him to support the effort to loosen the Bush rules.

Ronald M. Green, director of the Dartmouth College Ethics Institute, says he questions the moral logic of engineering special embryos for research. Sen. Arlen Specter (R-Pa.), who is undergoing cancer treatments, tries to illustrate the urgency he says is required to find a cure for diseases.