Donald Trump has enjoyed steady support from white evangelicals but new figures suggest even their loyalty is being tested by the government shutdown.

A new NPR/PBS NewsHour/Marist poll shows Trump's approval ratings within his supporter base down from one month ago as the longest government shutdown in US history drags into its 28th day.

And there is no end in sight as Congress and Trump continue to stand off over his demand for $5.7 billion to help fund a border wall between the US and Mexico.

The latest figures show that those who approve of the US president are down from 42 per cent in December to 39 per cent now.

The numbers disapproving have risen over the last month from 49 per cent to 53 per cent and the cause of that increase lies within his key supporter groups.

Republican support has dropped from 90 per cent approval and 7 per cent disapproval, to 83-to-10 per cent. Among suburban men, the past month has seen a fall from 51 per cent approval and 39 per cent disapproval to 42-to-48 per cent.

White evangelicals also appear to be growing tired of the stalemate, as approval in this group has dropped from 73-to-17 per cent to 66-to-23 per cent.

Lee Miringoff, director of the Marist Institute for Public Opinion, said it was the first time the president's base showed signs of cracking and it could cause problems for him in 2020. 

'The president has had his base and not much else and when you look ahead to the election ... he enters with a significant disadvantage. His re-election prospects would definitely be in jeopardy at this point,' he said.

As federal workers take the financial hit for the impasse in Washington, some churches are stepping in to help.

New Birth Missionary Baptist Church near Atlanta, Georgia, has given members affected by the shutdown nearly $300 each in financial assistance.

Church pastor Dr Jamal Bryant said the church was looking at other ways to give sustained support, like potluck dinners and gift cards, until the shutdown ends.

'When the government shuts down is when the church needs to be wide open,' Bryant told WSBTV.

'I ain't waiting on the Democrats or the Republicans,' he added.

Oak Grove Missionary Baptist Church in Memphis, Tennessee, has taken a similar approach.

Pastor Donald Johnson asked the church board to give whatever came in for the Sunday offerings to eight federal workers in the congregation. The church board then matched that sum to increase the assistance for the affected workers. The eight workers received $1,000 each from the money raised.

'I just realized people were going to be affected by it, and we didn't want to spend any time just arguing and fighting about what the president is doing,' said Pastor Johnson.