WASHINGTON -- As the Senate prepares to vote on House-passed legislation that would loosen President Bush's restrictions on federal funding for embryonic stem-cell research, Sens. Arlen Specter and Tom Harkin are fighting off an alternate approach by administration officials and some conservative Republican senators.

Specter, R-Pa., and Harkin, D-Iowa, view the option as a bid to derail support for their proposal by offering a measure to provide funding for new techniques still in early stages of development, but potentially more ethically palatable to some lawmakers.

The Stem Cell Research Enhancement Act, for which Specter and Harkin are Senate sponsors, cleared a major hurdle in May when it passed the GOP-dominated House with strong bipartisan support. That legislation would expand the number of embryonic stem-cell lines available to scientists by providing federal money for research on embryos left over from fertility treatments at in-vitro clinics, as long as the embryos would otherwise be discarded and the couple have given their consent.

But because Bush has threatened to veto that legislation, Senate Majority Leader Bill Frist, R-Tenn., and fellow Republican Sens. Tom Coburn of Oklahoma and Johnny Isakson of Georgia are crafting a bill they are referring to as the "third way" on stem-cell research -- one that Specter and Harkin fear could siphon off support from their bill.

Frist has not revealed details of the "third way" legislation. But aides expect that it will permit funding for new techniques that scientists are exploring to harvest embryonic stem cells without destroying embryos -- methods that were debated by scientists and ethicists whom Specter invited to testify before a Senate subcommittee yesterday morning.

One technique that several conservative senators are touting would let scientists extract a cell or several cells from an embryo (as one might for a biopsy) -- for development into an embryonic stem-cell line -- without destroying the embryo.

But Dr. James F. Battey, director of National Institute of Health's stem-cell task force, testified at the hearing that the technique so far has reportedly been successful only in creating mouse stem-cell lines.

Several other methods that the scientists and ethicists discussed were extracting stem cells from "dead embryos" left over from fertility treatments, reprogramming cells to coax them into functioning the way embryonic stem cells would, and a method known as Altered Nuclear Transfer -- a complicated procedure that involves removing an adult cell's nucleus and implanting it into an altered egg cell, so that stem cells are created without actually creating an embryo.

Techniques such as those were cited in a recent report by the President's Council on Bioethics, but scientists and ethicists testifying yesterday emphasized that many are still unproven.

Harkin said he believed that the interest in those techniques arises mainly from those interested in defeating the Specter-Harkin legislation, sponsored in the House by Reps. Mike Castle, R-Del., and Diana DeGette, D-Colo. Harkin said he believed that the White House was causing much of the pressure.

"The method we're discussing hasn't been published in a single scientific journal, it hasn't been cleared for peer reviews; it's only been tried in mice," Harkin said. "We're a long way from proving it works in human embryos.

"I'm all for these alternative methods; let's go ahead and pursue them," he said, "but not as a substitute -- not as some way of stopping what we're about to do."

When asked about that charge yesterday, White House spokesman Allen K. Abney said the Bush administration was committed to working with lawmakers to explore all possible avenues for stem-cell research that do not lead to destruction of human life and "would leave no stone unturned" in that effort.

Specter, who was diagnosed earlier this year with an advanced stage of Hodgkin's disease, a cancer of the lymph system, personally appealed to his legislation's supporters to keep focused on the extraction method that has already proven effective.