"It crosses a moral boundary that our decent society needs to respect, so I vetoed it," Bush said.
The legislation, passed by both the House of Representatives and the Senate, now returns to Capitol Hill, where it does not appear to have the two-thirds majority needed to overturn Bush's first veto since taking office more than five years ago.
Even conservative Republicans who generally oppose abortion are divided. Bush sees the research as destroying a human life, but others, including Senate Majority Leader Bill Frist of Tennessee and Sen. Orrin Hatch of Utah, say the embryos are slated for destruction anyway. Iowa Democratic Sen. Tom Harkein pledged to reintroduce the bill next year and said Bush's veto was a "shameful display of cruelty, hypocrisy and ignorance."