Lawmakers in the Democratic-led US lower house approved plans to expand government funding for human embryonic stem-cell research, defying a threatened veto by President George W Bush.
The bill, which requires Senate approval before it could land on Bush's desk, signalled the first major clash between the Republican president and the centre-left Democrats who won control of Congress in November elections.
The proposal passed the House of Representatives by a 253-174 vote on Thursday and was also expected to clear the Senate. It would open the door for further government funding of the research, but specific details about money would be determined later. Cutting across party lines, 37 Republicans joined Democrats in backing the expansion.
But the House vote fell short of the two-thirds majority that would be needed to override a Bush veto.
Bush, a born-again Christian who draws critical support from socially conservative Republicans, rejected a virtually identical bill in July that was passed by a Republican-led Congress. He said it "crossed a moral boundary".
Many scientists believe stem-cell research holds the promise of cures for wasting diseases such as Alzheimer's. But a restrictive US policy adopted by Bush in 2001 limits research to some 20 lines of human embryonic stem cells that existed at the time.
Bush opposes opening up federally funded research to new stem-cell lines because it would allow "intentional destruction of living human embryos for the derivation of their cells" under US law for the first time, a White House statement said.
"Destroying nascent human life for research raises serious ethical problems, and millions of Americans consider the practice immoral," the White House said.
Bush would veto the bill in its present form, the statement said.
"Researchers are now developing promising new techniques to produce stem cells similar in nature to those derived from human embryos, but not requiring the use of embryos," the statement said.