Next year's US presidential election is the most important in terms of evangelicals' influence since Ronald Reagan's terms, according to Ben Carson's special faith adviser.

Johnnie Moore, a former youth pastor at Liberty University who now works in television production in Hollywood, told Fox News: "Evangelicals are playing an incredibly relevant role in this election, in fact this is probably the single most important election with regard to evangelicals that we can remember, maybe since Ronald Reagan in the 80s."

Moore said: "We've never had a more signficant competition for this group of voters."

Carson's support among evangelicals has shown a sharp drop in recent polling, with Ted Cruz and Donald Trump tied on 24 per cent and retired neurosurgeon Carson on 19 per cent. Marco Rubio is on 13 per cent and Jeb Bush on only four.

Moore said that Carson was "the least political of all the candidates and he's certainly the most authentic Christian", adding that he "speaks our language, he is one of us".

He said that Carson's membership of the Seventh Day Adventist Church was not a factor in the slippage of evangelical support for him. "It has zero impact," he said, adding that there were "nuanced differences on secondary theological issues" between Adventists and mainstream evangelicals.