Support for Donald Trump has waned significantly among a key group of working-class voters during the longest government shutdown in US history, polls indicate.

The president’s approval rating among white Americans without university degrees has plummeted in the last month, according to separate surveys by Quinnipiac University and CNN.

The demographic is seen as crucial to Mr Trump, with two-thirds backing him in the 2016 election.

Fifty-three per cent of white voters with no degree approve of the president’s performance, down from 56 per cent on 19 December, found the Quinnipiac poll. His disapproval rating has increased from 37 per cent to 43 per cent, according to the survey.

CNN’s research put the same group’s support for Mr Trump at 45 per cent, down from 54 per cent a month ago. The rating is his lowest polled by a CNN survey of white Americans without degrees, by one per cent.

The CNN poll found 55 per cent of all Americans placed most of the blame for the government shutdown on the president, compared to 22 per cent who blamed Democrats in congress.

Forty-five per cent of white Americans without university degrees blamed Mr Trump, with 39 per cent blaming Democrats.

The president initially appeared eager to take responsibility for the deadlock which has left 800,000 federal government workers unpaid for more than a month, telling Democrats in a televised meeting in December: “I will be the one to shut it down ... I’m not going to blame you for it.”

Earlier this month, Mr Trump vowed to keep shuttered government departments closed for “months or even years” if House Democrats continue to refuse to sign off funding for his border wall.

But with frustration growing over the shutdown, which entered its 26th day on Wednesday, the president has tried to shift blame onto Democratic leaders Nancy Pelosi and Chuck Schumer.

“Nancy and Cryin’ Chuck can end the Shutdown in 15 minutes. At this point, it has become their, and the Democrats, fault,” he tweeted on Monday.