Paul Manafort was a key player in the early stages of the Trump presidential campaign.

He began by helping the campaign wrangle delegates for the Republican National Convention. And when campaign manager Corey Lewandowski was fired in June 2016, Manafort took the reins, with Rick Gates signing on as Manafort's deputy.

On Monday, Manafort and Gates pleaded not guilty to charges stemming from an investigation by Justice Department special counsel Robert Mueller. A grand jury indictment charged the two with conspiracy, money laundering, failing to register as foreign agents and making false statements for actions that took place before they joined the Trump campaign.

Mueller is looking into Russian meddling in the 2016 presidential election and has been investigating Manafort for his financial ties to a pro-Russia party in Ukraine. Manafort worked on behalf of Russian-backed Ukraine President Viktor Yanukovych, who was driven from power 2014.

Most of the charges in the 12-count indictment involve activity prior to Manafort joining the Trump campaign, although the indictment alleges that Manafort worked to hide his link to Ukraine while running the campaign.

Mueller's investigation is intended to determine whether anyone involved in the campaign helped Russian interests interfere with the election, but it is not limited to that topic.

Manafort, 68, is a long-time lobbyist and political operative who has worked for Republican campaigns dating back to Gerald Ford and Ronald Reagan. In 1980 he founded the consulting firm of Black, Manafort, Stone & Kelly.

In 1985, he began working with Filipino dictator Ferdinand Marcos, who won election in 1986 amid accusations of human rights violations and voter fraud. Marcos fled the country weeks later.

Around that time, Manafort also did public relations work for Angolan rebel leader Joseph Savimbi, whose military efforts drew the support of the U.S. government. Savimbi was killed by Angolan government forces in 2002.

More:Paul Manafort, aide Rick Gates charged with conspiracy, money laundering

More:Manafort conspired to mislead bankruptcy court, son-in-law charges

In 1988, Manafort worked with then-presidential nominee George H.W. Bush.

In the 1990s, Manafort started a new firm with partners Rick Davis and Matthew Freedman. In 1996, Manafort managed the failed presidential campaign of GOP candidate Robert Dole.

In 2004, Manafort began what multiple media reports cite as a decade-long, financially lucrative relationships with Oleg Deripaska, a billionaire with close ties to the Russian President Vladimir Putin.

Manafort's Virginia home was the subject of an FBI search this summer., and he was finally forced out by Trump in August amid questions about the campaign's tactics and his Russian ties.

In September, Manafort's estranged son-in-law accused him of conspiring to mislead a federal bankruptcy court about real estate investments. Jeffrey Yohai made the allegation in a case that centers on four troubled California real estate investments that collectively total millions of dollars. Manafort spokesman Jason Maloni declined to comment on the case.

Manafort's daughter, Jessica, filed for divorce from Yohai in March.