(FORTUNE Magazine) – WHEN SENATE MAJORITY LEADER Bill Frist backed a bill to expand the number of federally funded stem-cell lines a few weeks ago, he laid claim to something increasingly rare in America's polarized "embryo politics"--middle ground. Democrats and liberal pundits would have preferred more ambitious action, but they hailed his cautious, nuanced compromise. Many Republicans, caught between their desire to show support for this popular research and their fear of crossing the religious right (not to mention their President), were quietly grateful for the cover offered by Dr. Frist, who boasts both deep scientific knowledge and--at least for now--a 100% voting score from the National Right to Life Committee. So here's the next question: Having found this new ground, will Frist have to abandon it almost immediately to take on an even more contentious issue--cloning?

Outraged pro-lifers, from columnists Bill Kristol and Charles Krauthammer to Catholic bishops and the NRLC, are insisting that Frist must now yoke any Senate vote on extending federal support for new embryonic stem-cell lines to a ban on all forms of human cloning. A Senate bill to do that has been introduced in each of the past three Congresses by Senator Sam Brownback (R-Kansas). But Frist may be hoping to finesse this issue. In a recent FORTUNE interview, he said he continues to favor a total ban on cloning. But when asked if he supported Brownback's bill, he appeared to dodge. "I haven't taken a specific position on it," he said. "There's real concern among a number of people about the criminalization of researchers. But until I go back and see how he defines cloning, I'm not going to be able to comment intelligently on it."

Of course, a bill simply banning reproductive cloning in the U.S. would stampede through Congress faster than you can say Sarbanes-Oxley. The real fight is over therapeutic cloning, which promises to provide the most powerful tool yet for researching and developing "patient-specific, disease-specific" drug therapies (and potentially, cures) for scourges such as Parkinson's, Alzheimer's, and juvenile diabetes (for more, see "Fighting for Their Lives" on fortune.com). The procedure, also called SCNT (somatic cell nuclear transfer), involves taking nuclear material from a person's cell, injecting it into a nonfertilized egg, then inducing the egg to grow in a petri dish, creating a clone.

To grossly oversimplify, opponents of therapeutic cloning say the U.S. must ban the technology outright because experimenting on nascent life is a dangerous first step down a slippery slope toward "clone-and-kill embryo farms" (and yes, those cells are destroyed once they are created). That's what Brownback's bill would do by imposing jail terms of up to ten years and big fines on anyone creating an embryo through therapeutic cloning.

Proponents counter that (1) these cells aren't the same as human life created by sperm and egg, and (2) by simply banning the act of placing the cells created through therapeutic cloning in a uterus, we can draw a firm moral line across that slippery slope. That's what a Senate bill sponsored by Republican Orrin Hatch and Democrats Dianne Feinstein and Tom Harkin would do. (Like Brownback, they would also ban reproductive cloning.)

So what happens next? Frist, who is articulate and informed about the need to make new stem-cell lines eligible for federal funding, seems ready to push for Senate action on the House-passed bill to do that, even in the face of President Bush's promised veto. As for Brownback's bill, despite the conservative clamor, Frist apparently sees it as, well, less of an immediate priority. He may consider therapeutically cloned embryos "nascent life," as he told FORTUNE. But in a deeply split Senate, aides suggest he's not looking to push the cloning debate before its time. Sounds like a man trying--for now--to hold onto some hard-won middle ground. -- Rik Kirkland