Paul Manafort, President Donald Trump’s former campaign chairman, has pleaded not guilty to a variety of charges brought by Special Counsel Robert Mueller’s team.

Manafort has been the subject of a long-running investigation over his dealings in Ukraine several years ago – for which he didn’t file as a foreign agent until June 2017. But Mueller has incorporated that investigation into his own probe of Russian meddling in the 2016 election and possible collusion with Trump associates.

A federal judge has set a Sept. 17 trial date for Manafort.

What kind of foreign work did Manafort do?

Paul Manafort, former campaign chairman for U.S. President Donald Trump, departs after a bond hearing as part of Special Counsel Robert Mueller's ongoing Russia investigation, at U.S. District Court in Washington, U.S., December 11, 2017. - HP1EDCB19FMGO

Paul Manafort, former campaign chairman for President Trump, departs after a bond hearing as part of Special Counsel Robert Mueller’s ongoing Russia investigation, at a U.S. District Court in Washington.

 (Reuters/Joshua Roberts)

A GOP operative who worked for former Presidents Ronald Reagan and George H.W. Bush, Manafort reportedly began his work in Republican politics in the 1970s.

Eventually, Manafort was hired by former Ukrainian President Viktor Yanukovych, a controversial pro-Russia politician who was ousted from power twice. After Yanukovych was elected president in 2010, Manafort reportedly stayed on as an adviser and worked on other projects in Eastern Europe, including the Party of Regions political party.

Manafort also worked for Russian billionaire Oleg Deripaska. In 2005, Manafort allegedly came up with a plan to influence U.S. politics, business dealings and the media in order to “greatly benefit the Putin government,” according to The Associated Press.

Deripaska is a close ally of Russian President Vladimir Putin, and signed a $10 million annual contract with Manafort in 2006; they maintained a business relationship until at least 2009.

Financial records obtained by The New York Times indicated that Manafort was in debt to pro-Russian interests by up to $17 million prior to joining Trump’s campaign.

He also took more than a dozen trips to Moscow and frequently talked to Putin allies over a period of about 10 years, McClatchy reported. He traveled to Kiev at least 19 times in 20 months after the February 2014 removal of Ukraine’s pro-Russia leader.

How was Manafort involved with Trump’s campaign?

Manafort joined Trump’s presidential campaign in March 2016 to help wrangle delegates ahead of the Republican National Convention in Ohio, something he’d done for former President Gerald Ford.

Just two months later, Manafort became Trump’s campaign chairman. 

Manafort’s resignation from the campaign was announced on August 19, 2016, after The Times reported that he’d received $12.7 million in undisclosed cash payments from Yanukovych’s pro-Russian party between 2007 and 2012.

Along with Donald Jr., Trump’s eldest son, Manafort met with Russian lawyer Natalia Veselnitskaya in June 2016. She reportedly was said to have damaging information on Trump’s campaign rival, Hillary Clinton, which was “part of Russia and its government’s support for Mr. Trump.”

What has Manafort been charged with?

Former Trump campaign manager Paul Manafort, one focus of special counsel Robert Mueller's investigation into alleged Russian meddling in the 2016 U.S. presidential election, hides behind his car visor as he leaves his home in Alexandria, Virginia, U.S. October 30, 2017. REUTERS/Jonathan Ernst - RC1F89D175A0

Former Trump campaign chairman Paul Manafort, one focus of Special Counsel Robert Mueller’s investigation into alleged Russian meddling in the 2016 presidential election, hides behind a car visor as he leaves his home in Alexandria, Va., after being asked to surrender to federal authorities.

 (Reuters/Jonathan Ernst)

Along with his former business associate Rick Gates, Manafort was initially indicted in October on multiple counts that included: conspiracy against the U.S., conspiracy to launder money, false statements and failure to file reports of foreign bank and financial accounts.

Nearly four months later, on Feb. 22, the pair were hit with additional tax evasion and bank fraud charges. The additional charges involve much of the same conduct Manafort and Gates were initially charged with, but the amount of money Manafort is accused of laundering through offshore accounts increased to $30 million.

Manafort and Gates initially pleaded not guilty to the charges. However, Gates eventually pleaded guilty to federal conspiracy and false-statements charges on Feb. 23. After the plea, Mueller moved to drop the 22 bank and tax fraud charges against Gates, possibly suggesting that the former Trump campaign official is cooperating and providing good information to Mueller’s team. 

Mueller also accused Manafort of secretly paying former European politicians to lobby on behalf of Ukraine.

The charges against Manafort and Gates don’t relate to allegations of misconduct during Trump’s campaign.

Mueller pleaded not guilty on Feb. 28 to five counts from a new round of charges in Mueller’s probe. And on March 8, Manafort pleaded not guilty again to charges including tax evasion and bank fraud in a Virginia federal court. 

A federal judge assigned to the latest indictment from Mueller said in March that Manafort “faces the very real possibility of spending the rest of his life in prison.” 

What has the White House said?

Manafort’s alleged actions took place before he joined the Trump campaign, the president said on social media.

White House press secretary Sarah Sanders also downplayed Manafort’s involvement with the campaign in a press briefing. 

Fox News’ Samuel Chamberlain, Jennifer Earl, Jake Gibson, Zoe Szathmary and The Associated Press contributed to this report. 

Kaitlyn Schallhorn is a Reporter for Fox News. Follow her on Twitter @K_Schallhorn.