Evangelicals in the United States are rallying behind Donald Trump while it is the "nones" who back Hillary Clinton, according to new reserch.

The latest Pew Research Center survey finds that despite the professed wariness toward Trump among many high-profile evangelical Christian leaders, evangelicals are, if anything, even more supportive of Trump than they were of Mitt Romney at a similar point in the 2012 campaign.

Nearly eight in ten white evangelical voters say they would vote for Trump if the election were held today, including a third who "strongly" back his campaign.

Meanwhile, voters who describe their religion as "atheist," "agnostic" or "nothing in particular" are lining up behind Clinton, much as they supported Barack Obama in 2012.

Two-thirds of religiously unaffiliated registered voters say they would vote for Clinton if the election were held today.

"Considering both groups are quite large, the votes of white evangelical Protestants and religious 'nones' could be important to the outcome of the 2016 election," says Pew in its report.

White evangelical Protestants make up one-fifth of all registered voters in the US.

Religious "nones," who have been growing rapidly,  make up one-fifth of all registered voters.

The support for Trump among Christians is solid even though many evangelical leaders have suggested it is incompatible with evangelical principles and beliefs. In the survey, more than half of white evangelical voters say they are dissatisfied with the choice of presidential candidates.