Paul Manafort, the beleaguered chairman of Donald Trump's presidential campaign, resigned today after a staff shake-up this week that marginalised him in the team amid growing speculation about his links with Ukrainian politics.

"This morning Paul Manafort offered, and I accepted, his resignation from the campaign. I am very appreciative for his great work in helping to get us where we are today, and in particular his work guiding us through the delegate and convention process," Trump said.

"Paul is a true professional and I wish him the greatest success," the 70-year-old business tycoon said.

Manafort's role was reduced after Trump elevated two aides to senior positions on Tuesday, appointing Breitbart Newschief Steve Bannon as campaign CEO and Kellyanne Conway as campaign manager.

Manafort, who had once lobbied for pro-ISI and anti-India groups, had been recently attracting negative publicity for his alleged lobbying activities in Ukraine.

Manafort had joined the campaign after Trump's historic wins in the Republican primaries.

He successfully led the campaign in the Republican National Convention in Cleveland, Ohio, where Trump was formally nominated as the party's presidential nominee.

Trump's popularity rating had been falling down after the convention.

Manafort told Trump he was becoming a distraction and he wanted to end that, a Trump source was quoted as saying by CNN.

The resignation comes as the campaign seeks to correct course after weeks of damaging controversies and self-inflicted wounds, effectively evaporating Trump's steady footing against Democratic candidate Hillary Clinton in the polls and his post-convention bump.

Trump is now trailing Clinton in every major poll.