Donald Trump’s campaign chairman, Paul Manafort, suddenly resigned Friday amid a team shakeup and new reports about his pro-Russian work for a political party in Ukraine.

Trump announced Manafort’s departure in a statement.

“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. Paul is a true professional and I wish him the greatest success.”

Manafort officially took over the GOP nominee’s operation after Trump fired campaign manager Corey Lewandowski in June. But earlier this week, Trump suddenly elevated pollster Kellyanne Conway to campaign manager and Breitbart News executive Stephen Bannon to campaign CEO — an apparent demotion for Manafort.

That shakeup occurred after Trump struggled to regain his footing following last month’s political conventions. A series of firestorms, including Trump’s public feud with a Gold Star family and his provocative quip about “Second Amendment people,” have since roiled his White House bid. The most recent RealClearPolitics average of national polls gave Democrat Hillary Clinton a 6-point lead over Trump.

Manafort has also been battling weeks of critical scrutiny, from Yahoo News and other outlets, related to his ties to Russian political interests in Ukraine.

Just days ago, former Congressman Vin Weber, R-Minn., told Yahoo News Chief Investigative Reporter Michael Isikoff that Manafort recruited him in 2012 to lobby for the European Centre for a Modern Ukraine.

Weber said that he’d repeatedly asked Manafort for the names of the businesspeople backing the Brussels-based nonprofit, but that Manafort declined to supply them.

It turns out that the backers had close ties to the pro-Russian government of then Ukrainian President Viktor Yanukovych.

“It would be very hard to look at this entity and say it was not directed by the then Ukrainian government,” Adrian Karatnycky, a Ukraine expert at the Atlantic Council, told Isikoff. “It’s pretty clear they were running interference on sensitive issues on behalf of Yanukovych.”

According to reports, Manafort lobbied to gain positive U.S. press coverage for Ukraine’s pro-Russia officials in an attempt to sway public opinion.