Newly announced Democratic presidential candidate, former New York Mayor Michael Bloomberg speaks during a press conference to discuss his presidential run on November 25, 2019 in Norfolk, Virginia.

An longtime advisor to billionaire Democratic presidential candidate Mike Bloomberg has cut ties with a Ukrainian oligarch who once made a six-figure donation to President Donald Trump's now-defunct foundation.

The aide, Doug Schoen, confirmed to CNBC that he decided to not work with Victor Pinchuk, a businessman who owns a piping company and has investments in steel, real estate and television, after eight years in order to join the Bloomberg campaign.

"I do not represent Mr. Pinchuk anymore. And did this the moment Mike Bloomberg made clear his intentions to formally announce his candidacy for president," said Schoen, a veteran consultant and lobbyist who once worked as President Bill Clinton's pollster.

Pinchuk is also on the board of the Yalta European Strategy, or YES, a forum for discussing Ukraine's future in Europe.

Pinchuk has worked to have a relationship with Trump since the New York real estate magnate and reality TV star kicked off his White House run in 2015. Special counsel Robert Mueller was reportedly investigating a six-figure payment Pinchuk made to Trump's foundation.

Bloomberg, who has a net worth of over $55 billion, officially announced his candidacy on Sunday but has signaled to allies for weeks that he was actively considering it. A lobbying disclosure report says Schoen terminated his contract with Pinchuk on Nov. 6. Schoen noted that he has not discussed the decision with Bloomberg.

A spokesperson for Bloomberg did not return a request for comment. Pinchuk could not be reached.

Schoen has had a long relationship with Bloomberg. Schoen's private consulting firm touts its work for Bloomberg's 2005 mayoral run. Bloomberg served as mayor for three terms, spanning 2002 to 2013.

Schoen has earned millions working for Pinchuk since 2011. The Ukrainian businessman has a net worth of $1.4 billion, according to Forbes.

The latest lobbying report shows that Schoen made over $320,000 in a six-month period ending Oct. 31. That payment includes a $240,000 investment for "advisory/consulting services." It does not give details about those services.

Documents obtained by McClatchy show that in 2017 Pinchuk invited Trump and members of his administration to one of his annual YES meetings, although it appears no one from the White House attended. In 2015, when Trump was a candidate for president, the future president participated in a 20-minute question and answer session through a video conference call. Pinchuk introduced Trump, and Schoen sat next to the Ukrainian billionaire.

Trump's appearance came after Pinchuk donated $150,000 to the Donald J. Trump Foundation, which agreed to dissolve last year as New York state investigators worked to determine whether the president's namesake charity misspent donations.

The New York Times previously reported that Mueller was investigating the payment. Trump's former personal lawyer Michael Cohen, who is serving a prison sentence for charges related to the Mueller probe, told Congress that Trump told him to have Pinchuk make a donation to the foundation instead of paying him a speaking fee.

Earlier this month, a New York judge ordered Trump to pay $2 million to settle a suit by the state's attorney general alleging the president misused the charity to benefit his 2016 campaign.