In just the third veto of his administration, President Bush blocked an attempt by Democrats to federally fund embryonic stem cell research.
"If this legislation became law, it would compel American taxpayers for the first time in our history to support the deliberate destruction of human embryos," President Bush said.
But in an acknowledgment of growing public support and mounting political pressure for the research, Bush ordered government agencies to push for alternatives.
"We want to encourage science. We want to say we stand on your side in an ethically responsive way," President Bush said.
Standing behind the president was Kaitlyne McNamara, a spina bifida patient saved by doctors who grew a new bladder from her own cells. "And here she stands, healthy," said Bush.
But Democrats insist embryonic cells are the best hope.
"This has promise for diseases that affect 110 million Americans and we don't feel like we can just let that go," Rep. Diana DeGette of Colorado said.
And they are painting the president as out of touch.
"This is just one example of how the president puts ideology before science," Sen. Hillary Clinton of New York said.
Each side accused the other of political pandering.
"And I think you will find that the president's reverence for life is shared by a majority of the American public," White House spokesman Tony Snow said.
Democrats don't have the votes to overturn the veto, but still plan to continue the fight.