Donald Trump is continuing his efforts to woo the evangelical vote, releasing a video in which he holds up a Bible and promises his evangelical supporters that he will "never let you down".

The video was posted to Trump's Facebook page on Saturday, and has been watched over 976,000 times.

"I really appreciate the support given to me by the evangelicals," the Republican presidential candidate says in the clip.

"They've been incredible. Every poll says how well I'm doing with them. And you know, my mother gave me this Bible, this very Bible, many years ago, in fact it's her writing right here...it's just very special to me.

"And again, I want to thank the evangelicals. I will never let you down."

A caption accompanying the video notes the "warm embrace" given by evangelical Christians to Trump on the campaign trail.

So far, Trump has attracted significant evangelical support, with high-profile leaders including Liberty University's Jerry Falwell officially endorsing his campaign.

However, the latest poll released by the Des Moines Register and Bloomberg on Saturday shows that evangelical support for Trump may be waning.

Ahead of the Iowa caucuses today, the poll showed that though Trump is leading overall with 28 per cent, he is not faring quite so well with evangelical conservatives in the state.

A third of those who identified with that group said they will vote for Ted Cruz, while just 19 per cent aligned themselves with Trump.

Results of a LifeWay survey last week showed that Trump may also be failing to gain traction among church leaders in the US.

1,000 senior church leaders were asked: "If the 2016 presidential election were held today, for whom would you vote?"

Ted Cruz won the race among pastors who identified as Republican, with 29 per cent of those polled saying they would vote for him. More than a third (39 per cent) were undecided, but one in 10 said they would vote for Ben Carson, while eight per cent chose Marco Rubio. Just five per cent named Donald Trump as their top choice.