In his statement, McCain at first claimed to support ESC research. However, he said "clear lines should be drawn that reflect a refusal to sacrifice moral values and ethical principles for the sake of scientific progress" -- a qualification that disturbed many scientists and bioethicists with its ambiguity.

McCain also took a harder line than the Bush administration with somatic cell nuclear transfer, better known as therapeutic cloning -- a cutting-edge process that could some day provide personalized embryonic stem cell therapies. Though currently legal, McCain would outlaw the technique.

The new stance is an abrupt reversal for the Arizona senator. As recently as 2007, McCain appeared to favor embryonic stem cell research more strongly than most of the Republican party, especially its most religiously conservative members. "I believe that we need to fund this," he said during a presidential candidates' debate in May 2007.

Since then, he's become steadily cagier in his support, courting Kansas Sen. Sam Brownback, an ardent opponent of all ESC research, and avoiding discussion of ESCs in favor of alternative cell types. Those familiar with the debate interpreted McCain's latest platform, which framed his support in the language of research opponents, as a signal that President Bush's research-limiting policies may continue.

"I am researching SCNT and so would be considered a criminal if McCain gets his way," said the Harvard researcher Daley. "It's a sad society that starts criminalizing legitimate science."