Diana DeGette, one of the bill's chief sponsors in the House of Representatives, said: "We ... intend to continue bringing this up until we have a pro-stem cell president and a pro-stem cell congress."

 

Hillary Clinton, the senator from New York and a 2008 presidential hopeful, said: "This is just one example of how the president puts ideology before science, politics before the needs of our families, just one more example of how out of touch with reality he and his party have become."

 

Executive order

 

"President Bush isn't fooling anyone with this executive order, and the fact that it doesn't change the policy adds insult to injury for the millions of patients who suffer every day"

Sean Tipton, president of the Coalition for the Advancement of Medical Research
In an effort to blunt criticism of the veto, Bush issued an executive order to encourage scientists to work with the federal government to derive new methods to obtain stem cells without harming human embryos.

 

Tony Snow, the White House press secretary, said: "This is, certainly not an attempt to muzzle science. It is an attempt, I think, to respect people's conscience on such an issue."

 

Critics called the presidential order a sham and said he should have signed into law the stem cell bill, which polls show is backed by more than 60 per cent of American voters who see the measure as a way to combat a host of illnesses.

 

Sean Tipton, a pro-stem cell advocate, said: "President Bush isn't fooling anyone with this executive order, and the fact that it doesn't change the policy adds insult to injury for the millions of patients who suffer every day."

 

Tipton leads the Coalition for the Advancement of Medical Research, a non-partisan advocacy group that represents more than 100 patient and scientific organisations.

 

'Morally offensive'

 

Bush and other critics condemn the proposed legislation as morally offensive because it would lead to destruction of human embryos to derive stem cells.

 

The White House said: "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.

 

"The president has made it clear ... that he will not allow the nation to cross this moral line."

 

Supporters note that the legislation would only permit scientists to use embryos left over from fertility treatments, which would otherwise be discarded. They also say it could clear the way for possible medical breakthroughs that could help millions of people suffering with debilitating diseases.

 

Shortly after Bush took office in 2001, he allowed federal funding on 78 stem cell lines then in existence. Most turned out to be of limited use to scientists, who have urged the president to lift his restrictions.

 

Democrats promised to push to expand federally funded stem cell research in winning control of congress from Bush's Republican party last year, and it is likely to be an issue again next year when US voters elect a new congress and president.