Paul Manafort was in deep with the same Pro-Russia people in the Ukraine as Joe Biden's son, Hunter. I suspect he was 'planted' into the Trump campaign by the same actors who staged the elaborate, but typical KGB/FSB-style "Trump-Russia collusion" hoax. The person who introduced Manafort to team Trump was Thomas J. Barrack.
Paul Manafort
Chairman of Donald Trump’s 2016 campaign
In February 2016, Manafort approached Donald Trump through a mutual friend, Thomas J. Barrack Jr.
He pointed out his experience advising presidential campaigns in the United States and around the world, described himself as an outsider not connected to the Washington establishment, and offered to work without salary.[49]
In March 2016, he joined Trump’s presidential campaign...”
In August 2016, Manafort’s connections to former Ukrainian President Viktor Yanukovych and his pro-Russian Party of Regions drew national attention in the US, where it was reported that Manafort may have received $12.7 million in off-the-books funds from the Party of Regions.[56]
On August 17, 2016, Donald Trump received his first security briefing.[57]
The same day, August 17, Trump shook up his campaign organization in a way that appeared to minimize Manafort’s role.
It was reported that members of Trump’s family, particularly Jared Kushner who had originally been a strong backer of Manafort, had become uneasy about his Russian connections and suspected that he had not been forthright about them.[58]
Manafort stated in an internal staff memorandum that he would “remain the campaign chairman and chief strategist, providing the big-picture, long-range campaign vision”.[59]
However, two days later, Trump announced his acceptance of Manafort’s resignation from the campaign after Steve Bannon and Kellyanne Conway took on senior leadership roles within that campaign.[60][61]
https://en.wikipedia.org/wiki/Paul_Manafort#Chairman_of_Donald_Trump’s_2016_campaign
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Thomas J. Barrack Jr
-snip-
He [Thomas J. Barrack] denies a quote attributed to him in the 2018 book Fire and Fury that he said Trump was, “not only crazy” but “stupid”.[38]
In a 2017 Washington Post article, Barrack commented on Trump’s inflammatory rhetoric and proposals to ban immigrants from certain Muslim countries and put up a border wall with Mexico. “He’s better than this,” he said.[39]
Barrack was interviewed during the Special Counsel investigation into Russian interference in the 2016 election, in particular regarding Paul Manafort, Rick Gates, Konstantin Kilimnik, Cambridge Analytica, the Trump campaign, the Trump transition team, and the financing of the Trump inauguration.[40][41]
https://en.wikipedia.org/wiki/Thomas_J._Barrack_Jr.#Political_activity