Facing a tough re-election battle, US President Donald Trump has replaced his campaign manager, on a day when opinion polls showed that his Democratic rival Joe Biden has taken a double digit lead over him.

"I am pleased to announce that Bill Stepien has been promoted to the role of Trump Campaign Manager," the president said in a Facebook post on Wednesday night.

Stepien, Trump's former second-in-command, replaces Brad Parscale as the president's campaign manager, some 16 weeks before the November 3 election.

The shakeup comes as the Trump campaign has sought to revive the momentum amid a coronavirus pandemic that wreaked havoc on a once booming US economy and a national reckoning over race following African-American George Floyd's custodial death.

Parscale is reported to have been sidelined in recent weeks after Trump's comeback election rally in Tulsa, Oklahoma, flopped.

Parscale had boasted that more than one million people registered to attend Trump's rally, but less than 6,200 showed up at the arena.

"Brad Parscale, who has been with me for a very long time and has led our tremendous digital and data strategies, will remain in that role, while being a Senior Advisor to the campaign," Trump said.

"Both were heavily involved in our historic 2016 win, and I look forward to having a big and very important second win together," he added.

"This one should be a lot easier as our poll numbers are rising fast, the economy is getting better, vaccines and therapeutics will soon be on the way, and Americans want safe streets and communities!" Trump said.

According to Fox News, Parscale ran Trump's digital advertising in the 2016 US presidential polls and was credited with helping bring about the Republican candidate's surprise victory.

Stepien has been in politics for years, working for former New Jersey governor Chris Christie and serving as Trump's national field director in 2016, the news channel said.

Meanwhile, former vice president Biden has opened up a 15-point lead over President Trump, according to a new Quinnipiac University national poll.

It also spells out bad news for the President on the economy, his most important selling point for reelection, CNN reported.

A majority of registered voters (52 per cent) report they will support Biden in the general election, with 37 per cent opting for Trump, a wider margin than in Quinnipiac's June poll, when 49 per cent backed Biden and 41 per cent supported Trump.

An NBC/WSJ poll published on Wednesday also finds Biden leading Trump with an increasing margin, Biden with 51 per cent support among registered voters over Trump's 40%.

In the NBC/WSJ June poll, Biden lead Trump with 49 per cent to 42 per cent.

The poll also finds growing disapproval of the president's job performance -- with a notable shift on the economy where reviews of Trump's performance have turned negative.

The economy had been the only issue on which Trump earned a net positive rating in Quinnipiac's polling, but now, 44 per cent approve while 53 per cent disapprove of his performance.