Washington DC

US senators voted last week, for the second time in nine months, to lift restrictions on federal funding for human embryonic stem-cell research — even though they expect President George W. Bush to veto the decision again.

Tom Harkin (left), Arlen Specter and Orrin Hatch have pushed hard for changes to the stem-cell law. Credit: A. C. GLENN/UPI/NEWSCOM

So why the repeat of last July's political meltdown? In part, senators are positioning themselves for next year's congressional elections and for the one political contest that will bear enormously on future US stem-cell policy: the presidential race of 2008.

In that race, stem-cell research “is going to be a very big issue, maybe right up alongside the war”, says Daniel Greenberg, a veteran Washington science commmentator.

Jockeying began in the Senate on 11 April, when members voted 63–34 in favour of the Stem Cell Research Enhancement Act of 2007. The bill allows federal funding for research on stem cells that are derived from embryos left over at fertility clinics and already slated for destruction. It is similar to one passed by the Senate last summer and then vetoed by Bush (see Nature 442, 335; 2006). And no sooner had last week's vote been cast than the president issued a statement promising to do the same again. “This bill crosses a moral line that I and many others find troubling,” he said. “If it advances all the way through Congress to my desk, I will veto it.”

In the presidential race, stem-cell research will be a very big issue, maybe right up alongside the war.

The House of Representatives is expected to vote on the bill in the next few weeks. If it passes the House and is vetoed by Bush, it is unlikely that Congress will muster the two-thirds majority necessary in both houses to override the presidential veto. Three Democrats missed last week's Senate vote; all would almost certainly have voted in favour, bringing the total votes in favour to 66 — one shy of a two-thirds majority. The margin in the House is larger: a House vote on a similar bill in January failed by 32 votes to reach a veto-proof majority (see Nature 445, 134–135; doi:10.1038/445134a 2007).

So Bush's current policy, which dates from 9 August 2001, looks almost certain to remain in place for now. This limits federal funding to research on stem-cell lines that existed on that date, a number that has dwindled to 21.

But whoever assumes the presidency in January 2009 could implement his or her new stem-cell policy with “a wave of a hand” by using an executive order, says Jonathan Moreno, a professor of medical ethics at the University of Pennsylvania in Philadelphia. That means that voters on both sides of the issue are closely scrutinizing the positions of the leading presidential contenders.

Senator Sam Brownback (Republican, Kansas) spoke out against the bill last week and is the only declared presidential candidate who aggressively opposes stem-cell research. But he is a second-tier contender. Three other, stronger, Republican hopefuls are treading delicately as they try to court moderates without losing votes on the right.

Rudolph Giuliani, the Republican former mayor of New York City, has thus far skated around the stem-cell issue, saying that new technology needs to be taken advantage of while life also needs to be respected. Mitt Romney, a Mormon, originally supported stem-cell research as Republican governor of Massachusetts. Then in February 2005, he announced that he opposed the cloning of embryos for research purposes and would support criminal sanctions for those doing so. Senator John McCain (Republican, Arizona) voted in favour of the bill that passed last week.

So, too, did two leading Democratic contenders, Hillary Clinton of New York and Barack Obama of Illinois, both of whom were among the bill's many co-sponsors. And former senator John Edwards (Democrat, North Carolina) strongly supports federal funding for the research; after last week's Senate vote, he issued a statement urging Bush not to veto the bill.

Amongst all this political manoeuvring, Senate conservatives also bought themselves some political cover. Last week, the Senate passed the Hope Act, which Bush has promised to sign if it is also passed by the House. This allows federal funding for research on stem cells that are derived from embryos that the bill terms “naturally dead”. The bill passed by a vote of 70 to 28, with Republicans the main supporters.

Sponsored by Norm Coleman (Republican, Minnesota), the bill defines “naturally dead” as “having naturally and irreversibly lost the capacity for integrated cellular division, growth, and differentiation”. Critics, however, note that there is no scientific standard for determining what is a viable embryo.