WASHINGTON -- Two senior associates of US President Donald Trump went to jail on Tuesday fallen to separate federal criminal cases linked to a special investigation into Russian interference in the 2016 presidential race.

The exceptional series of events marked a watershed moment in Trump's presidency, under legal scrutiny from multiple angles as Special Counsel Robert Mueller looks at the role he and his campaign aides might have played in undermining the US election system.

Michael Cohen, Trump's longtime personal attorney and a veteran of the Trump Organization, pleaded guilty to tax evasion, bank fraud and campaign finance violations– including paying hush money to Trump's mistresses in the late hours of the 2016 race "for the principal purpose of influencing the election" and "in coordination and at the direction of a candidate for federal office."

The plea effectively implicates Trump in a conspiracy to break election law, raising new legal and political challenges for the embattled president.

Cohen paid off at least two women, Karen McDougal and Stephanie Clifford, to quiet them on "alleged affairs" they conducted with Trump. He will serve jail time for up to five years having pleaded guilty to eight felony charges, but posted bond until his sentence hearing, scheduled for December.

Cohen's case was farmed out of Mueller's office to the Southern District of New York, which handled the case after federal law enforcement raided his home and hotel room in April. "He worked to pay money to silence two women who had information that he believed would be detrimental to the campaign," Robert Khuzami, a deputy US attorney, said on Tuesday.

Lanny Davis, Cohen's lawyer, said in a statement on Tuesday that his client sought to end the probe for the sake of his family– and to finally come clean on Trump's activities.

"Today he stood up and testified under oath that Donald Trump directed him to commit a crime by making payments to two women for the principal purpose of influencing an election," Davis wrote on Twitter. "If those payments were a crime for Michael Cohen, then why wouldn't they be a crime for Donald Trump?"

Meanwhile, Paul Manafort, Trump's former campaign chairman, was also found guilty of eight felony counts in a separate federal trial in Virginia. The jury there found him guilty of five counts of tax fraud, two counts of bank fraud and one count of hiding foreign bank accounts, altogether carrying up to 80 years in prison.

Manafort has deep ties to pro-Russian interests in Ukraine and led Trump's campaign when the Republican Party made changes to its platform favorable to Moscow. Cohen, meanwhile, led Trump's years long effort to build a Trump Tower in the Russian capital.