WASHINGTON (Reuters) - President George W. Bush vetoed legislation on Wednesday that would expand federally funded embryonic stem cell research, triggering an uphill battle to override him in the Democratic-led U.S. Congress and likely pushing the issue onto the 2008 election agenda.

President Bush speaks during a meeting with Republican members of the House of Representatives in the Oval Office, June 20, 2007. From (2nd L-R) are Congressmen John Boehner (R-OH), Paul Ryan (R-WI) and Jeb Hensarling (R-TX). REUTERS/Kevin Lamarque

Two-thirds majority votes would be needed in the Senate and House of Representatives to overcome Bush’s opposition and make the bill law. Backers admitted they were short of support. A few dozen of Bush’s Republicans in Congress back the measure.

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

“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,” Bush said. “I will not allow our nation to cross this moral line.”

Advocates such as the nonprofit Society for Women’s Health Research said Bush’s decision was the unethical one, especially as the embryos that would be used are left over from fertility treatments and destined to be discarded.

“There should be no ethical debate between throwing away embryos that already exist and using them in the scientific quest for treatments and cures of fatal and life-threatening diseases,” the group said in a statement.

“It is a tragedy to allow embryos to be wasted and discarded, when we could be exploring their unique potential to alleviate human suffering.”

Embryonic stem cells are the source of every cell, tissue and organ in the body. Scientists study them to understand the biology not only of disease, but of life itself, and want to use them to transform medicine and find cures for such debilitating illness as Parkinson’s, cancer and diabetes.

In addition to vetoing such legislation for the second time in two years, 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. He announced no new funding.

The Campaign for the Advancement of Medical Research, a lobby group made up of medical and advocacy groups, called the executive order a sham.

“The new approaches addressed by the order are still in the early stages of development and appear to already be eligible for National Institutes of Health funding,” the group said in a statement.

WIDE BACKING

Polls show the stem-cell bill is backed by more than 60 percent of American voters.

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“Once again, the president has ignored the will of the American people, of leading medical researchers, and of a bipartisan majority of the Congress,” said House Speaker Nancy Pelosi, a California Democrat.

Republican Sen. George Voinovich of Ohio backed Bush, saying, “We can and must find cutting-edge techniques to cure disease and ease suffering without destroying life.”

The National Right to Life Committee also praised Bush.

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

Democrats promised to push to expand stem cell research in winning control of Congress last year, and it appeared certain to be an issue again next year when U.S. voters elect a new president and Congress.

An estimated 400,000 embryos are unused and frozen at fertility clinics. A poll released on Wednesday showed about 60 percent of parents of those embryos would donate them for stem cell research. Just 22 percent would donate them for adoption by other couples, which is what Bush says should be done with them.

Bush noted that scientists this month reported that in tests with mice they had made progress on alternative ways to produce stem cells without destroying embryos -- an advance, which if it works in humans, could ease ethical concerns.

Those scientists cautioned there was no guarantee of quick success with human cells, and urged passage of the bill.