The US House of Representatives has passed a bill backing embryonic stem cell research, marking a major challenge to President George W Bush.
Opponents refuse to condone the destruction of any embryo
The stem cell bill was among the top priorities for the Democrats, who took control of Congress last week, but Mr Bush has vowed to veto it.
Advocates of stem cell research say it could lead to cures for diseases such as Alzheimer's and Parkinson's.
Mr Bush says the research would destroy human life in the name of science.
The bill was passed by 253 to 174, but fell short of a two-thirds majority needed to overcome the veto.
"Today, by passing legislation to expand stem cell research, the House gave voice to the hopes of more than 100 million Americans and their families," said House Speaker Nancy Pelosi.
"With today's strong bipartisan vote, we now challenge President Bush to join members from both sides of the aisle in supporting the hope of stem cell research."
Mr Bush used his presidential veto to overturn a similar judgement by the Republican-controlled Congress last year.
It was the first time in his presidency that Mr Bush refused to sign into law a bill approved by Congress.
Polls suggest most Americans back the research.
Opponents of the bill say their taxes should not fund research which involves the destruction of embryos.
Its supporters maintain that the embryos used for research, that come from multiple embryos generated by couples trying to produce a pregnancy using in vitro fertilisation, would otherwise be discarded.