Evangelicals – and white evangelicals in particular – are planning to vote for Donald Trump over Hillary Clinton in droves. Both candidates suffer from low approval and 'likeability' ratings and many evangelicals are planning to abstain or vote for a third candidate as a protest. But in Clinton's case, the issue that makes her absolutely unelectable is that she is pro-choice – in favour of a woman's more or less unrestricted right to choose to abort her baby.

For most US evangelicals and Roman Catholics, life begins at conception. This is not a view evangelicals have always held – the Southern Baptist Convention in 1971 called for legislation to allow abortion under conditions such as rape, incest, severe foetal deformity, or damage to the emotional, mental, and physical health of the mother. It later expressed regret for its stance. After the crucial Roe v Wade ruling in 1973 that legalised abortion, even such a doughty conservative as Walter Criswell welcomed it, saying: "I have always felt that it was only after a child was born and had a life separate from its mother that it became an individual person," he said, "and it has always, therefore, seemed to me that what is best for the mother and for the future should be allowed."

Neither is it the case that abortion has always been a political dealbreaker for evangelicals, or decided along party lines. Republican president Ronald Reagan was personally pro-life but when he was governor of California he signed into law the Therapeutic Abortion Act to reduce the number of back-street abortions.

But abortion became a key political battleground with the rise of the religious right and its ideological identification with the Republican party. And according to Randall Balmer, a Columbia University professor and author of Thy Kingdom Come, this was a deliberate policy rather than a spontaeous revulsion at the consequences of Roe v Wade.

In his book, subtitled An Evangelical's Lament, Balmer says most evangelical leaders did not respond to Roe v Wade. He recalls a meeting at which one of the founders of the Moral Majority movement, Paul Weyrich, spoke animatedly about the formation of the Religious Right in the late 1970s. It came about, he said, as a result of efforts by Jimmy Carter to deny segregationist colleges tax-exempt status. Weyrich, corroborated by others, told Balmer conservatives held a conference call to discuss their strategy and find a unifying issue. "Several callers made suggestions, and then, according to Weyrich, a voice on the end of one of the lines said, 'How about abortion?' And that is how abortion was cobbled into the political agenda of the Religious Right," says Balmer.

There are two issues here. One is whether abortion was cynically used by the right as a way of getting evangelical Christians onside in a struggle for political influence. On Balmer's evidence, it was.

But the other issue is about the thing itself. Whatever the origins of the abortion lobby, most evangelicals have been convinced by the argument that life begins at conception and that abortion is, to one degree or another, profoundly wrong. This is a line argued passionately by campaigners such as Francis Schaeffer, Harold Brown and C Everettt Koop in the 1970s, and particularly in Koop's bombshell book Whatever Happened to the Human Race? (1979). Their campaigns, and Koop's book in particular, helped persuade a generation of evangelicals that abortion is profoundly evil – and they haven't changed their minds. 

Pro-lifers are on one side of an argument that continues to split the country as a whole. According to a Gallup poll from May 2016, 50 per cent of Americans are pro-choice – referring to the "woman's right to choose" slogan – while 44 per cent are pro-life. However, Gallup said it was the first time since 2008 that the pro-choice side's lead was statistically significant. It also found that 51 per cent of Americans believe abortion should only be legal under certain circumstances, which would include laws such as parental consent and ultrasound viewings.

However, in spite of the great support for the pro-choice position, attempts to limit abortion access have been successful at a state level across the country. According to the pro-abortion Guttman Institute, 334 abortion restrictions have been enacted by states since 2011. These include restricting late abortions, refusing to provide healthcare services from public funds, requiring mothers to have ultrasound scans before an abortion, restricting insurance coverage and banning abortions for sex or race selection or genetic abnormality. Other attempts to restrict abortions have failed in the courts.

Proponents of these laws want to make them even tougher and abortion even harder. (Republican presidential nominee Donald Trump even suggested punishing women who had abortions before backtracking on his remarks.) They point to a consistent downward trend in abortion statistics: in 2014 there were 954,000 abortions in the US – a staggeringly high figure, but one that continues a consistent fall in numbers since 1990, when numbers peaked.

Against that, pro-lifers point to the Democratic Party's campaign platform, which commits to championing the controversial Planned Parenthood organisation and "vows to oppose, and seek to overturn, all federal and state laws that impede a woman's access to abortion, including by repealing the Hyde Amendment" ­– a reference to a longstanding prohibition barring the use of federal funds to pay for abortions. It's the great fear of pro-lifers that what they regard as gains under previous administrations and state law might be undone if Clinton takes office.

Defenders of Clinton such as Tony Campolo say that while she is pro-choice, she is personally committed to reducing the number of abortions in the US – he claims by 50 per cent. Campolo says she will address this by reforming childcare, wages and labour laws. According to Campolo: "Abortions will not be ended through legislation. We must find ways to keep women who want to give birth from being driven by economic forces into curtailing their pregnancies."

By its nature, abortion is a polarising subject. Any talk of reducing the number of abortions is resisted by liberals, who say that by calling for it to be "safe, legal and rare", as Clinton used to do, is to stigmatise those who choose it. But for pro-lifers – only just a minority in the US – the idea that the decision to end a life, for reasons that may seem trivial or unworthy, could simply be reduced to "a woman's right to choose" is unconscionable.

In the end, evangelicals voting in the US election on the basis of candidates' abortion policies will need to be guided by a hard-headed consideration – which candidate is likely to do more to reduce the numbers? On the present showing Clinton is very unlikely to persuade them it's her.

Follow Mark Woods on Twitter: @RevMarkWoods