Donald Trump must increase his support from Christians if he is to stand a chance of winning another election, according to a leading pollster.

In an article for the American Culture and Faith Institute, executive director George Barna, who founded the Barna Group polling organisation, analyses Trump's Christian support base.

Barna says 57 per cent of all self-identified Christians voted for Trump as president, including 53 per cent of Christians describing themselves as 'born-again' – a fact he describes as 'incredible since born-again voters deemed his character detrimental and they were not sure if they could trust him to stick to his freshly-minted conservative views'.

Since the election his favourability rating has declined slightly among born-again Christians, from 58 per cent in February 2017 to 54 per cent in January 2018.

However, he says Trump's achievements on health care, immigration, abortion and other evangelical priorities have played positively with born-again believers, with generally favourable responses.

However, says Barna, 'his inability to get Congress on board with other pressing issues certainly has Trump's most loyal constituents – Christians – on edge. The fact that his favorability rating has declined, albeit slightly, in the past year within that segment ought to be a red flag to him.'

He warns: 'If he loses a mere five percent of the backing of born-again Christians, he is guaranteed to be a one-term president. Elections are a numbers game, and the numbers clearly show that Trump cannot afford the slightest erosion from his base of supporters.'

A further challenge for Trump, he says, is that 'notional Christians', representing around 40 per cent of voters, are less enthusiastic in their support and 'will not support the incumbent in 2020 unless Trump completes more of his original agenda'. Barna notes that 'It was the first presidential election of the last eight in which Notionals sided with the GOP candidate over the Democratic option.'

Trump is likely to face 'intense' opposition in 2020, and 'that, in turn, means he will need to increase his levels of support from Christians – among both the born again and Notional varieties – if he is to have a reasonable chance of getting re-elected,' Barna concludes.