President Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE is troubled by the possibility that special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation into possible collusion between the Trump campaign and Russia could expand to include an examination of Trump's and his family's finances.

The Washington Post reported late Thursday that the president has been particularly upset by the notion that Mueller could access Trump's personal tax returns, which he has repeatedly declined to release to the public.

As part of his investigation into the Kremlin's role in the 2016 presidential election and whether members of Trump's campaign coordinated with Russian officials or operatives to influence the race, Mueller is also looking at possible financial ties between campaign members and Russian interests.

ADVERTISEMENT

Bloomberg reported on Thursday that the special counsel has begun to look at Trump's business transactions, as well as those of his associates.

In an interview with The New York Times on Wednesday, Trump accused Mueller of running an investigation riddled with conflicts of interest and said the special counsel would cross a line if his probe expanded to include the Trump family's finances.

Trump bucked a decadesold tradition during the presidential race by refusing to release his personal income tax returns.

He initially said his decision not to do so was because he was under audit by the IRS. He later said that the public "doesn't care" about his tax returns, and that only journalists were interested in obtaining them.

Every president since Jimmy Carter has released his returns.