By Daniel Dombey, Andrew Ward and Guy Dinmore

Abortion is one of the most incendiary themes in US politics, and the Roman Catholic Church has taken an increasingly tough line in the debate.

The issue may be particularly resonant in states such as Florida, Michigan, Pennsylvania and Ohio, where the Democrats have found it hard to win over key blue-collar Catholics in the past.

“It’s significant and it’s perhaps a tribute to the US that the Holy Father would accept to come in an election year because, generally, that would be one of the conditions that would be avoided for a pontifical pastoral visit,” said Archbishop Timothy Broglio, a former papal envoy who heads the Archdiocese for the Military Services USA.

He said that while the Pope had made it clear that because of his age he would not undertake many trips, he had decided to visit the US after having been asked to address the United Nations in New York.

“I do not doubt that he will seek to promote peace and respect for the human rights of all – from conception to natural death,” added the archbishop, while stressing that the Pope’s remarks “should not be construed in a partisan manner”.

During the 2004 presidential race, several Catholic bishops warned John Kerry, the Democratic nominee and a Catholic, that they would not give him communion because he favoured maintaining abortion’s legal status. The Pope – then Cardinal Joseph Ratzinger – appeared to back their stance in a memo he sent to US bishops.

Last year, the US bishops issued a “call to political responsibility” that held that “a Catholic cannot vote for a candidate who takes a position in favour of an intrinsic evil, such as abortion or racism, if the voter’s intent is to support that position”. It added that supporting pro-abortion candidates would “be permissible only for truly grave moral reasons”, and that such political choices “may affect the individual’s salvation”.

Briefing on the papal visit, Archbishop Donald Wuerl of Washington acknowledged that it could be difficult to separate the Pope’s message wholly from the context of the US election

“There will be all kinds of things going on around him and they will be the filter through which much of what he says will be seen,” he said. “That’s going to be the challenge he will face.”

Mr McCain hopes his consistent pro-life record will help win over Christian conservatives who are wary of his moderate stance on some other issues.

But he has offended some Catholics by embracing an evangelical preacher known for his anti-Catholic views. John Hagee, leader of a big Texas “megachurch”, has referred to the Catholic church as “the great whore” and called it a “false cult system”.

Mr McCain has condemned Mr Hagee’s anti-Catholic views, but refused to reject his endorsement.

Catholics form the single largest religious denomination in the US, but not since John F. Kennedy became the first Catholic president in 1961 have they voted as a unified bloc.

Catholics traditionally voted Democrat, but many defected to the Republican party during the Reagan era – becoming so-called “Reagan Democrats” – because of disillusionment with the Democrats’ drift to the left on social issues.

In 2004, John Kerry, the Democrats’ first Catholic presidential nominee since Mr Kennedy, narrowly lost the Catholic vote to Mr Bush, a Methodist.

Once concentrated in the north-east and Midwest, the Catholic population has been transformed over recent years by nationwide growth in the heavily Catholic Hispanic population.

Mr Bush won strong support among Hispanic Catholics by appealing to their socially conservative in-stincts, although many have since been alienated by the Republicans’ tough stance on immigration.

A veteran observer of the Vatican told the Financial Times that the papal trip would be a test for the Pope after his successful visit to Turkey last year.

He noted that the Pope’s support of the UN, where he will praise the universal declaration of human rights, was not to the liking of many conservatives, and that on issues such as social policy, climate change and the death penalty, his position was more to the left.

“He is a paradox, not neatly fitting the compartments of the contemporary world,” said the observer.

The Financial Times