President George W. Bush’s veto on Wednedsay of embryonic stem cell legislation marks a turning point for a president who has until now been able to dictate what the Republican-controlled Congress has sent to the White House for him to sign.

Mr Bush’s veto – the first in his five and half years in office – sent the bill to expand federal funding of the controversial research back to the House of Representatives, where backers of the measure were not able to muster the two-thirds majority needed to override his decision. 

The legislative flurry would leave intact an executive order Mr Bush signed in 2001 that limited federal funding to the stem cell lines that existed at the time.

But the episode offers clear evidence that the sensitive nature of the debate – and concerns among Republicans about the November midterm elections – have shifted the ground in Washington.

“This is the first time a Republican-controlled Congress has sent to the president a bill with a moral basis that is opposite of his,” said Michael Franc, a congressional expert at the Heritage Foundation, a conservative think-tank.

With Republicans in control at the White House and on Capitol Hill, there has been room for negotiation in past disputes over issues such as spending on highway or military projects.

“I have never believed it made any sense for a Republican Congress to send a bill to the president that he was going to veto, [so] he could look good, we could look bad, or vice versa,” said John Boehner, majority leader in the House. “So on virtually hundreds of bills, their concerns were dealt with to put the bill in a position where he could sign it.”

But compromise on the morally charged research, which opponents equate with the taking of human life, was not possible.

“It crosses a moral boundary that our decent society needs to respect, so I vetoed it,” Mr Bush said.

Public opinion polls show Mr Bush at odds with a majority of Americans, and Democrats have already begun to use the issue in the midterm election campaign.

The decision by Republican leaders to let the legislation proceed – backed by most Democrats and many Republicans – reflected pressure from moderates, eager to assert their independence from Mr Bush and the conservative wing of the party.

“It’s a sign of an every-man-for-himself attitude, mostly among the moderates,” said Mr Franc. “They are saying, ‘we have to vote the way we need to, to get re-elected in November’.”

But analysts said they did not expect the split on stem cells to launch a series of vetoes from Mr Bush.

“He likes to see the Republican party working in unison,” said Kathryn Dunn Tenpas, a political scientist at the University of Pennsylvania who studies presidential vetoes.

The White House has issued 141 veto threats since Mr Bush was sworn in, the White House said. But not since Thomas Jefferson has a president spent so long in office without using his veto power. Even presidents whose parties also controlled both houses of Congress have made frequent use of the authority, averaging two vetoes per year, according to Ms Tenpas.

“This is such a charged social issue, he couldn’t back down on his threat,” she said. “I’m sure they’d rather not have done it.”