The Senate debate this week on stem cell research is freighted with consequences – for the future health of humanity and for the politics of 2006 and 2008.

To deal with first things first, when the House passed stem cell legislation in May 2005 by a margin of 238-194, 50 Republicans joined with almost all the chamber’s Democrats in support. The House bill would expand the supply of embryonic stem cells by allowing federally funded research on cells derived from embryos created for fertility treatments or donated from in vitro fertilization clinics. Those embryos would have to be in excess of the clinical needs for infertility treatments and otherwise destined to be discarded. They would have to be obtained with written consent and acquired without any payment to the donors.

Despite all those precautions, opponents say the destruction of the days-old embryos is the taking of human life – the equivalent of murder. President Bush shares that view, and that is why he issued an executive order in 2001 limiting federal funding to the few lines of stem cells already in existence. He has threatened his first veto if the Senate approves the House bill.

Nonetheless, Senate Majority Leader Bill Frist, himself an opponent of abortion, has yielded to the pleas of the legislation’s supporters and scheduled it for Senate debate. Frist, a physician, has said, “It isn’t just about faith, it’s a matter of science,” thus siding with the researchers who say that the embryonic cells are far more useful for their experiments than those derived from adults or from other animals.

To accommodate critics, Frist also has scheduled two other bills affecting stem cell research. One would direct the National Institutes of Health to push ahead on research that someday could yield stem cells without destroying embryos, yet be as flexible in their potential as embryonic cells. And the other would ban fertilizing eggs in women or in test tubes specifically to yield embryos for research.

Debate on the three bills will be limited to two days, and a filibuster-proof 60 votes will be required to pass any of them. Proponents of the House bill say they will muster that number and they are not fighting either of the other measures, so they should pass easily.

The odds are that President Bush will carry through on his pledge to issue his first veto on the stem cell bill. Given the narrow margin of Republican control, it is unlikely that both the House and Senate would produce the two-thirds majority needed for an override. So the issue will probably carry over to the fall elections.

Stem cells are already an issue in several of the hottest Senate races. In Missouri, Republican Sen. Jim Talent was assailed by opponents of stem cell research when he announced he was withdrawing as a co-sponsor of restrictive legislation sponsored by Sen. Sam Brownback of Kansas. Talent then came out against a November state referendum funding a stem cell research center, a measure backed by important parts of the Missouri business community and by his Democratic opponent, State Auditor Claire McCaskill. To highlight the issue, Democrats invited McCaskill to give the party’s radio response to Bush on Saturday.

The issue is also stirring in New Jersey, where Democratic Sen. Robert Menendez is in favor of research and state Sen. Thomas H. Kean Jr., his Republican opponent, says he favors embryonic stem cell research but has voted against such bills. And it is looming as an issue in Maryland, where Lt. Gov. Michael Steele, the Republican contender, created a fuss by comparing stem cell research to the experiments the Nazis conducted during the Holocaust.

In Tennessee, all three Republicans contending for the Senate seat that Frist is vacating in January have declared their opposition to the bill he has endorsed. Meanwhile, Rep. Harold Ford Jr., the certain Democratic nominee for the seat, voted for the bill when it passed the House.

In all these states, and in the nation as a whole, polls show public opinion supports expanded stem cell research. One voice has been particularly convincing – that of Nancy Reagan. She persuaded several anti-abortion Republicans in the House to vote for the bill by arguing that stem cells offer the promise of curing Alzheimer’s disease, which killed her husband, and many other dread afflictions.

This week, Sen. John McCain of Arizona, an opponent of abortion and a prospect for the 2008 Republican nomination who has recently been courting the religious right, told me that Mrs. Reagan had lobbied him recently and he would be supporting the bill.

She has made the case that voting for stem cells is doing it for the Gipper.