White evangelical Christians favour Donald Trump over Hillary Clinton by a margin of 55 percentage points, according to a poll from Fox News.

The poll also finds that among all voters, when a total of four candidates are taken into account, Clinton's lead over Trump has narrowed to just two per cent.

Clinton is polling 41 percent compared to 39 per cent for Trump's, with nine per cent opting for the Libertarian Gary Johnson and just four per cent for the Green Party's Jill Stein.

When the figures are examined to look at just Clinton versus Trump, her lead comes out at 48 to 42, a lead of six points that has fallen from the 10-point lead she was polling earlier last month.

Fox reports that Clinton is backed by blacks, with a 68 point lead over Trump when all four candidates are taken into account. She also has a 15 point lead among women and a 19 point lead among Hispanics.

Trump has a 13 point lead from whites and men, a 23 point lead among whites without a college degree and a 55 point lead from evangelical Christians.

Of Republicans, 74 per cent back Trump, compared to 81 per cent of Democrats who back Clinton.

Among the leading Christian pastors who have backed Trump are the president of Liberty University, Jerry Falwell Jr, and televangelist Robert Jeffress. Falwell recently compared Trump to Winston Churchill.

Peter Wehner, senior fellow at the conservative think tank, Ethics and Public Policy Center, writes in National Review that Falwell is "damaging Christian witness" by supporting Trump.

Wehner says: "For those of us of the Christian faith, the fact that Falwell is viewed by many as an Evangelical leader makes it that much worse. We have been pained by the harm that a previous generation did to Christian public witness because of partisan, reckless, and graceless comments. Now we have this."