As Pope Benedict XVI arrived in the United States yesterday, hundreds of thousands of Catholic voters were preparing to play a pivotal role in next week’s Democratic primary in Pennsylvania between Hillary Clinton and Barack Obama.

The Vatican is sensitive to the timing of the Pope’s visit, saying he does not want to get “mixed up in the internal, local political process”, while warning candidates against trying to “instrumentalise” the pontiff by making him a political tool.

But with Catholics expected to account for more than a third of the turnout in next week’s important primary, as well as being a key target group for both Democrats and Republicans in the elections this autumn, the Pope’s every utterance is certain to be scrutinised closely.

Mrs Clinton, a Methodist who gets strong support from white working- class Catholics and Hispanics, has been swift to spot an opportunity, issuing a statement saying America was “blessed” to be hosting the Pope.

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“Not only is he the spiritual leader of America’s great Catholic community, he is a strong and effective voice for the cause of peace, freedom, and justice as well as the fight against poverty and disease,” she said.

“His visit to the United States this week should be a very impactful one.” She got 63 per cent of the Catholic vote in Ohio last month and hopes for a similar margin in Pennsylvania after Mr Obama’s remarks suggesting that “bitter” people in such states “cling” to gun ownership or religion.

In recent days Mr Obama has done his best to draw back from the impact of such comments, insisting he was not seeking to “demean” faith, which is a “bulwark, a foundation, when other things aren’t going well”.

His campaign has employed Catholic outreach directors in Pennsylvania, encouraged volunteers to call fellow churchgoers on his behalf, and held small round-table “listening meetings” with members of the Church.

Yesterday, he followed Mrs Clinton with his own statement, saying: “The theme of Pope Benedict’s journey, ‘Christ Our Hope’, offers comfort and grace as well as a challenge to all faith communities to put our faith into action for the common good.

“It will not only be Catholics who are listening to the Holy Father’s message of hope and peace; all Americans will be listening with open hearts and minds.”

Despite Mr Obama’s stance in favour of abortion rights, he has been backed by Senator Bob Casey Jr, the fiercely “pro-life” Democratic senator for Pennsylvania, who has built on the links established by his father, Pennsylvania’s former Governor, with the Catholic working class.But there remains suspicion towards Mr Obama, not least because of the remarks made by his controversial Protestant black liberationist pastor, the Rev Jeremiah Wright.

The Catholic vote, once part of the Democratic bedrock, is now increasingly diverse, a reflection, perhaps, of the way that policy positions advocated by the Vatican cut across party lines. The Church supports ending the war in Iraq and abolishing the death penalty, but is also strongly opposed to abortion, stem-cell research and gay marriage.

Bill Clinton carried the Catholic vote easily in 1992 and 1996, but in 2000 Al Gore was only 3 per cent ahead of George W. Bush among this group. Four years later, the Methodist Mr Bush won 52 per cent among Catholics, against 47 per cent for John Kerry, a Catholic himself.

Mr Kerry’s defeat came after a bitter controversy over whether Catholic politicians such as Mr Kerry who support abortion rights should be denied Communion. Among the church leaders advocating this course was Cardinal Joseph Ratzinger, who became Pope Benedict XVI less than a year later.

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