Despite the constitutional separation of church and state, religion plays a big and sometimes decisive role in politics in America, where levels of belief and regular worship are far higher than those in Europe.
"Non-Christians are concerned that they will be excluded from the process," said Ahmed Rehab, a spokesman with the Council on American-Islamic Relations.
"I welcome faith values if they inspire candidates to do good things. But I worry if it is used as a litmus test to include someone in political participation."
About 75 per cent of the US population, long a melting pot of immigrants from around the world, identifies itself as Christian, according to several estimates.
That is a huge but divergent source of potential votes for Republican and Democratic candidates in their long contest for the nomination to run for the White House in the November election.
US politicians are not shy of talking about their religion and regularly appear in church.
In recent decades, part of the American political drama has been scripted by the "religious right" - mostly white evangelical Protestants united by strong opposition to abortion and gay marriage who have been a key base of support for the Republican Party.
Republican hopeful Mike Huckabee, who scooped up strong evangelical support but whose campaign is fading ahead of next Tuesday's nominating contests across the country, is a Baptist preacher who peppers his speeches with Biblical allusions.
Mitt Romney is a Mormon who was moved to address questions about his faith in a speech in December. John McCain has long sought to smooth relations after including leaders of the religious right among those he called "agents of intolerance" during his failed presidential bid in 2000.
The leading Democratic presidential contenders have also been open and candid about their faith.
That faith, and that of the Republican candidates, is Christian, although candidates have also spoken about the need for religious tolerance.
