Donald Trump's campaign chairman Paul Manafort has resigned in the wake of a campaign shake-up and revelations about his work in Ukraine.
In a statement issued as he arrived in Louisiana to tour the flood-ravaged state, Trump said Manafort offered his resignation Friday morning. The billionaire called Manafort "a true professional".
"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.
Manafort's resignation comes a day after The Associated Press reported that confidential emails from Manafort's firm contradicted his claims that he had never lobbied on behalf of Ukrainian political figures in the US.
Trump's son, Eric Trump, told Fox News in an interview that will air Sunday that his father worried questions about Manafort's past were taking attention away from the billionaire's presidential bid.
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"I think my father didn't want to be, you know, distracted by whatever things Paul was dealing with," Eric Trump said.
Emails between Manafort's deputy, Rick Gates, also a top Trump adviser, and the lobbying firm Mercury LLC showed that Manafort's firm, DMP International LLC, directly orchestrated a covert Washington lobbying operation on behalf of Ukraine's then-ruling political party.
The effort included not just legislative outreach but also attempts to sway American public opinion and gather political intelligence on competing lobbying efforts in the US.
The emails obtained by the AP do not describe details about the role of Manafort, Gates' boss at the firm. But current and former employees at Mercury and a second lobbying firm, the Podesta Group, some of whom spoke on condition of anonymity because they are subject to non-disclosure agreements, told the AP that Manafort oversaw the lobbying efforts and spoke by phone about them.
Manafort and Gates never registered as foreign agents for their work as required under federal law.
Under the US Foreign Agents Registration Act, people who lobby on behalf of foreign political figures or political parties must provide detailed reports about their actions to the Justice Department's counter-espionage unit. A violation is a felony and can result in up to five years in prison and a fine of up to $US250,000 ($A325,205). Also on Friday, Ukrainian anti-corruption investigators released copies of handwritten ledgers detailing possible cash payments from Ukrainian political figures to Manafort totaling more than $US12 million ($A15.6 million). Manafort denies receiving those payments.
Earlier this week, Trump brought in a new campaign chief executive and campaign manager following a disastrous stretch in which the New Yorker committed a series of errors and fell behind Democratic nominee Hillary Clinton in both national and battleground state preference polls.
The new campaign manager, Kellyanne Conway, had described Manafort and Gates as part of a new "core four" atop the Trump operation with her and CEO Stephen Bannon. But Manafort's role in the campaign had been significantly marginalised under the new leadership structure.
