Personal religious convictions again bubbled to the surface of the presidential campaign with a supporter of Hillary Clinton referring to the Islamic faith of Barack Obama's father and Mike Huckabee releasing a new advertisement featuring a “floating cross”.

Mr Obama, a leading Democratic candidate, has increasingly invoked Scripture in speeches and talked about his “personal relationship” with Christ after false rumours spread across the internet that he was a secret Muslim.

The former senator Bob Kerrey, a notoriously outspoken politician and failed presidential candidate in 1992, stirred the pot when he endorsed Mrs Clinton and then proceeded to compliment Mr Obama for his understanding of Islam.

“I like the fact that his name is Barack Hussein Obama, and that his father was a Muslim and that his paternal grandmother is a Muslim,” Mr Kerrey said. “There's a billion people on the planet that are Muslims, and I think that experience is a big deal.”

When challenged about his comments, Mr Kerrey insisted that his intention had been to praise Mr Obama. He suggested that there was a “smear campaign” and that people were “acting as if he's an Islamic Manchurian candidate [because] he spent a little time in a secular madrassa” as a child in Indonesia.

Mr Obama, a committed Christian, has suggested that his time spent abroad would strengthen his foreign policy — but there is no evidence whatsoever that he has ever attended a madrassa.

Mr Kerrey's remarks appeared to jar with Mrs Clinton's campaign theme this week, which is now emphasising her positive message rather than the criticisms that she made of Mr Obama's character and policies in the past fortnight.

Support from evangelicals is regarded as particularly important in early voting states such as Iowa and South Carolina, where Mr Huckabee has leapt into the lead in the Republican race ahead of Mitt Romney, whose Mormon faith is regarded as a cult by some Christians.

In his latest advertisement Mr Huckabee says — with the tune Silent Night playing in a festively decorated room — that for all the politics going on at this time of year “what really matters is the celebration of the birth of Christ”.

The camera pans across a background showing a bookcase that forms a white cross behind his right shoulder.

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