Paul Manafort, advisor to Donald Trump, is seen on the floor of the Quicken Loans Arena at the Republican National Convention in Cleveland, Ohio, July 19, 2016.

Paul Manafort, Donald Trump's campaign chairman, spent decades advising Republican presidential candidates, but it's his time between campaigns which is making uncomfortable headlines at the moment.

Before joining Trump's presidential campaign in March this year, Manafort had worked as a consultant for the Party of Regions, the political party led by ex-Ukrainian President Viktor F. Yanukovych. He was also an advisor on the U.S. presidential campaigns of Gerald Ford, Ronald Reagan, George H.W. Bush and Bob Dole.

On Sunday, a New York Times report claimed that secret ledgers showed $12.7 million in undisclosed cash payments designated for Manafort from the Yanukovych administration. Manafort called the report "unfounded, silly and nonsensical" in a statement Monday morning. He added that he had never taken an "off-the-books cash payment" or worked for the governments of either Ukraine or Russia.

Corey Lewandowski, who was replaced by Manafort as Trump's campaign manager in June, tweeted a link to the article with no comment on Sunday night – which helped increase speculation of division in the Trump camp.

Tweet 1

On Saturday, Trump railed against NYT reporters as "dishonest people" over a story about divisions in his campaign, and threatened to revoke their credentials to cover the campaign.

Yanukovych was ousted in 2014, the first in a series of events which threatened to destabilize the region which included the occupation of the Crimea region of Ukraine by pro-Russian separatist forces.

Manafort's well-documented role as an advisor to a Ukrainian administration regarded as pro-Russian has already raised eyebrows, particularly given comments by Trump lauding Putin's leadership, dismissing the U.S.'s commitments to smaller NATO members, and his call for Russia to hack opponent Hillary Clinton's missing emails.

"The question is what more info the Ukrainian authorities have on Manafort's time in Ukraine, and how damaging any of this is for the Trump campaign," Timothy Ash, head of CEEMEA desk strategy at Nomura, wrote in a research note on Monday.