|PIC1|Presumptive Republican presidential nominee John McCain has one ace up his sleeve in his bid to woo disgruntled conservative Christians: his unflinching opposition to abortion rights.

His likely Democratic opponent in the November White House election, Barack Obama, firmly supports abortion rights.

Few other big issues cut so clearly across partisan lines in the United States, a point underscored by McCain and Obama's positions on it. And analysts say while both candidates must be careful they may need the issue to stir their party's bases.

In McCain's case that would be the evangelical Christians who account for one in four US adults and comprise a key base of support for the Republican Party - to such an extent that few analysts think he can win the presidency without them.

"Religious conservatives may not be wildly enthusiastic about McCain but they can point to his pro-life stance as reason to stay on board," said Matthew Wilson, a political scientist at Southern Methodist University in Dallas.

The Arizona senator's position on the issue distinguished him in the early stages of the Republican contest from former New York Mayor Rudy Giuliani, whose support for abortion rights dismayed conservative Christians and led to threats to form a third party if he had secured the nomination.

That signalled abortion was a line in the sand that this vital wing of the Republican Party would not cross and secured endorsements for McCain from leading conservative Christians such as Republican Kansas Senator Sam Brownback, whose own run for the nomination faltered.

Influential evangelicals like Focus on the Family's James Dobson, whose radio show reaches millions, have expressed their displeasure at McCain's past support for stem cell research and his failure to back a federal ban on gay marriage.

COMPARED TO SLAVERY

But nothing unites evangelicals like their opposition to abortion, which many compare to the anti-slavery movements of the past - a comparison that raises the moral stakes and suggests they will not back down on it.

Polls suggest the issue is becoming even more entrenched in conservative Christian culture.

An analysis of surveys from 2001 to 2007 by the Pew Forum on Religion & Public Life found that young white evangelicals between the ages of 18 and 29 were even more conservative on the issue than their elders.

It found 70 per cent said they were in favour of making it more difficult for a woman to get an abortion compared with 55 per cent of older white evangelicals and 39 per cent of young Americans overall.

McCain's stance also appeals to centrist evangelicals, who have been attracted to him by his opposition to abortion combined with his call for action on climate change and his resolute condemnation of the use of torture by US forces.

But his trump card with evangelicals could be a joker if he plays it badly in his bid to woo centrists and independents.

"For the Republicans it is a wedge issue because their right wing is very vocal on it. To bring it up at all you either risk the wrath of the right or you risk sounding too extremist to the middle," said David Epstein, a political scientist at Columbia University.

OBAMA'S HAND?

Is this an advantage for Obama?

"The Democrats can take a pro-choice position and...appeal to their base and to the middle," said Epstein.

Allen Hertzke, director of Religious Studies at the University of Oklahoma in Norman, said it could help Obama secure support from some of the white women who voted in droves in the Democratic nominating contests for Senator Hillary Clinton, who is running behind Obama for the party's presidential nomination.

But the issue is not a clear-cut one along gender lines.

"Some of the white working class women especially Catholics who supported Clinton are also pro-life and if abortion becomes salient it could hurt Obama among this group," he said.

Hot-button issues like abortion and gay marriage were important in 2004 when President George W Bush got close to 80 per cent of the votes cast by white evangelical Protestants.

However, white Catholic women who oppose abortion but voted for Clinton may well be focused on the dire state of the US economy, which Hertzke said was a clear plus for Obama.

Analysts said both sides could also pitch the issue to their bases as a struggle over the composition of the US Supreme Court whose justices are appointed by the president.

The anti-abortion rights movement has long had its eye on the big prize - a decisive conservative majority which would overturn the 1973 Roe v Wade decision granting women the right to an abortion.

McCain has reiterated that he would appoint such justices; for the Democratic base it is seen as vital that the tide of conservative appointees on the bench be rolled back.

Opinion polls consistently show that the parties' starkly different opinions on the issue are not held by large swathes of the middle in America. They show most Americans broadly support abortion rights but are less comfortable with the procedure in the later stages of pregnancy.