NEW YORK (AFP) - Donald Trump’s campaign chairman, Paul Manafort, resigned on Friday (Aug 19) under fire over a Ukrainian corruption investigation and after being sidelined by a recent personnel reshuffle as the Republican nominee languishes in the polls.

The departure of the smooth-talking strategist, who has advised Republican presidential candidates going back to Gerald Ford, follows weeks of damaging US media reports about a White House campaign in crisis.

“This morning, Paul Manafort offered, and I accepted, his resignation from the campaign,” Trump said in a statement, thanking him for “his great work” and hailing him a “true professional.”

Trump had recruited Manafort, who formerly advised pro-Kremlin former Ukrainian president Viktor Yanukovych and a host of foreign dictators, to head off what was then a potentially contested Republican convention.

It had marked Manafort’s first return to top-level Republican politics in 20 years – having been reportedly passed over in 2008 by John McCain, who was allegedly alarmed in part over his ties to Yanukovych.

But it seems it was an appointment that both Trump and Manafort came to regret.

Manafort was credited with trying to turn the brash-talking 70-year-old New York billionaire, who has never previously held elected office, into a figure more palatable to the general electorate.

Yet a series of controversies and colossal missteps, including a protracted row with the Muslim American parents of a soldier killed in Iraq, saw the Republican tank in the polls.

His Democratic rival Hillary Clinton now leads on average 47.2 per cent to Trump’s 41.2 per cent, according to Real Clear Politics, and is ahead in virtually every swing state.

But as the US press published story after story of a campaign in crisis, of staff tearing their hair out with their boss’ seeming inability to stay on message, Trump batted aside suggestions that he should change tack.

On Wednesday, he appeared to sideline Manafort further in appointing a right-wing news executive, Stephen Bannon, as campaign chief executive and promoting veteran pollster Kellyanne Conway to campaign manager.

The personnel reshuffle saw Manafort retain his position as chief strategist, at least on paper, but the move was widely seen as a demotion.

That came two days after Manafort was thrust into the headlines in connection with a Ukrainian corruption investigation, shining an unwelcome spotlight on his past as a lobbyist for rogues and dictators.

The head of Ukraine’s newly formed National Anti-Corruption Bureau, Artem Sytnyk, said that more than US$12 million (S$16 million) was earmarked for payment to Manafort from 2007 to 2012, although it was not clear if he received the money.

Manafort denied any wrongdoing, saying he had “never received a single ‘off-the books cash payment,’” or worked for the governments of Ukraine or Russia.

His departure now appears to coincide with a new era in the Trump campaign. On Friday, the ticket released its first television ads, equating Clinton with disorder and illegal immigration.

“In Hillary Clinton’s America, the system stays rigged against Americans,” claims the US$4.8 million ad, which is being aired in the battleground states of Florida, North Carolina, Ohio and Pennsylvania.

“Syrian refugees flood in. Illegal immigrants convicted of committing crimes get to stay, collecting social security benefits, skipping the line. Our border open.” In contrast, “Donald Trump’s America is secure,” it says. “Terrorists and dangerous criminals: kept out. The border: secure. Our families: safe.” Clinton has already spent US$61 million on television ads, according to NBC television.

On Thursday, Trump also took many by surprise in for the first time expressing “regret” for having said “the wrong thing” and causing “personal pain” – apparently alluding to recent missteps – at a campaign rally in Charlotte.