(FORTUNE Magazine) – By endorsing in late July a limited expansion of stem-cell research, Majority Leader Bill Frist, the Senate's only doctor, opened up some rare political daylight between himself and President Bush. But while religious conservatives and fellow travelers like Bill Kristol thundered at his apostasy--and liberals at the New York Times hailed his cautious step toward the political center--Frist insisted, in an inter- view with FORTUNE's Rik Kirkland, that his surprise move was based purely on science and conviction. (For the full text of their conversation, see fortune.com.)

Why did you give this speech at this time?

The political pundits are saying, "He's flip-flopping" or "He's positioning himself [to run for President in 2008]" or "He's destroyed himself for whatever he does in the future." I put all that aside. Because of my prominence as majority leader and as a physician and scientist who is also pro-life, I thought it very important that my 99 colleagues in the Senate understand my views. Making ethical decisions while advancing science has been a big part of my life--the 150 heart and lung transplants that I've done involved issues such as defining brain death, dealing with the scarcity of donors, what is life, what is death. Now that we have a five-week recess, I wanted to put the issue on the table so people will best be prepared to legislate when we come back to the floor in September.

Is there a big difference between your position and the President's?

The one thing that he handled differently [in 2001, when Bush announced his stem-cell policy] is that he restricted the number of cell lines by establishing a specific cutoff date, which some people said was arbitrary. That gave us 78 cell lines. At the time I thought we needed about 150. But 78 was not unreasonable, and since the President had opened the door to federally funding embryonic stem-cell research in a way that was morally and ethically consistent with my principles, I strongly endorsed his policy.

So what's different now?

What's different is the fact that those anticipated 78 cell lines proved to be only 22. And of those 22, probably less than 50% are sufficiently stable to be used for research purposes today. I've talked to the ten major scientists in this field and talked as well to scientists who don't want to use embryonic cells. Almost everybody says that at this point in time, the number and quality of cell lines that are federally supported is insufficient in terms of allowing the science to progress. But I continue to insist upon having an ethical construct to derive those new cell lines.

Explain that construct.

I believe we should not be creating living human embryos solely for the purpose of experimentation. Our efforts to derive embryonic stem cells today should be focused on the 400,000 embryos [currently frozen in IVF clinics], 300,000 of which will otherwise be thrown away, discarded, and destroyed. Let's say you want to generate another 200 new cell lines. Using today's technology, you're talking about using perhaps only 500 of the 300,000 embryos that are going to be thrown in the wastebasket anyway.

What about so-called therapeutic cloning, or somatic cell nuclear transfer (SCNT), which many scientists think holds the greatest immediate promise for using stem cells to treat disease?

That falls under two of the ten principles I declared in 2001, which included banning embryo creation for research and also banning human cloning. Many scientists will say, "Well, if it's less than 14 days, it really doesn't matter: That's not when life begins." Or "If it's not in a woman's uterus, it's not life." I believe that the embryo does have moral significance, and that arbitrary cutoff dates of 14 days or 21 days are just that--arbitrary.

Given that logic, your conservative pro-life critics say, Why create any stem cells from embryos? Aren't there alternatives?

I believe the federal government should put increased interest and funding in this arena [reprogramming adult stem cells and the like] because it for the most part eliminates the predominant ethical concern that the only way today to obtain stem cells is through the death and destruction of the embryo. But for the next one to three years, all the studies say, it is unlikely that these theoretical techniques will be able to derive stem cells in a clinically useful fashion.

Any chance the President might change his mind about his promised veto if the stem-cell bill you now support passes?

I can't speak for the President about this. I'll continue to talk with him about it. But in terms of the future, I just don't know.