US President Donald Trump's job approval rating remained at 42 per cent in the latest Gallup poll, unchanged from last month's figure and similar to the result in July.

The latest poll, conducted between August 31-September 13 and released on Wednesday, found 56 per cent of Americans disapproving of the job Trump has been doing, consistent with his scores since June, reports Xinhua news agency.

Views of Trump were sharply divided along party lines, with approval among Republicans ticking up by 2 percentage points from August to 92 per cent, while 36 per cent of independents approving of his what he has done as President.

Barely any Democrat – 4 per cent – supported the president.

Incumbent Presidents seeking re-election have usually enjoyed over 50 per cent of job approval rating at this point heading into the presidential race, and Trump now trails his predecessor Barrack Obama's 49 percent in 2012, George W. Bush's 52 per cent in 2004 and Bill Clinton's 60 per cent in 1996.

Trump was ahead of the 39 per cent approval for George H.W. Bush in 1992 and 37 per cent for Jimmy Carter in 1980, the last two Presidents having served only one term, respectively.

The Gallup poll came as Trump was still lagging behind his Democratic rival Joe Biden by mid to high single digits nationally and by narrower margins in swing states, polling data of Realpolitik showed.

The Gallup poll surveyed 1,019 adults and has a margin of error of 4 percentage points.

As both Trump and Biden were busy on campaign trails in recent weeks, Vice President Mike Pence said Florida, Arizona and Minnesota are the focus of the Trump-Pence ticket in order to win the 270 electoral votes needed for their second term.

"Florida's of great importance. Arizona's of great importance. We're going to make sure we continue to campaign in those states," Pence said in an exclusive interview with The Hill news website on Wednesday.

"We're actually looking at expanding the map now. I've been campaigning in Minnesota. So has the President," he added.