US President Donald Trump has replaced his reelection campaign manager as he faces a gap in the polls against presumptive Democratic nominee Joe Biden with less than four months to polling.

Mr Trump said on Facebook he would replace campaign manager Brad Parscale with Bill Stepien, who has been the deputy campaign manager.

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

"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!"

Mr Parscale, whom Mr Trump said would shift to a role focused on digital strategy, was blamed internally for the botched Tulsa, Oklahoma, rally last month that drew a much smaller-than-expected crowd.

Brad Parscale looks out where supporters are arriving for Donald Trump's rally in Tulsa

It led to a coronavirus outbreak that forced Mr Parscale and other campaign officials who were there to self-quarantine for two weeks.

Trump advisers have also privately leveled criticism that the campaign lacks a central message.

But they said Mr Trump shared the blame because he lacked discipline on how to communicate their message and agenda for a second, four-year term.

Mr Trump trails Mr Biden by ten percentage points among registered voters in the latest Reuters/Ipsos poll, which has shown voters turned off by his handling of the coronavirus pandemic.

The virus has also derailed his normal strategy of holding massive campaign rallies.

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