It's the issue that seems to rile him the most, and Donald Trump has now lashed out at "unlimited presidential harassment" after the Democrats announced a new probe into his finances.
The President blasted House Intelligence Committee head Adam Schiff on Twitter, saying the Democrat was "going to be looking at every aspect of my life, both financial and personal, even though there is no reason to be doing so."
In a 2017 with the New York Times, Trump declared an investigation into his family's personal finances was a "red line". That line has just been crossed.
The committee chair on Wednesday launched a sweeping investigation into allegations of money laundering and "financial compromise", vowing the probe would go "beyond Russia".
Trump charged that the Democrats had started afresh after finding "zero Russian Collusion", but the probe will also continue investigating the country's actions during the 2016 election and contact with Trump campaign team.
Schiff told reporters the scope would allow the committee to "investigate any credible allegation that financial interests or other interests are driving decision-making of the President or anyone in the administration" and "whether any foreign actor has sought to compromise or holds leverage, financial or otherwise, over Donald Trump, his family, his business, or his associates."
He said that would apply "to any credible allegations of leverage by the Russians or the Saudis or anyone else", following the committee's first meeting under the new, Democrat-controlled House of Representatives.
The wide-ranging investigation, which could involve other congressional committees, would also look at whether Trump or his associates have "sought to influence US government policy in service of foreign interests" whether they have obstructed the various investigations.
Trump fired back that the California congressman had "no basis to do that", in comments to reporters at the White House on Wednesday.
"He's just a political hack trying to build a name for himself," said Trump. "It's called presidential harassment and it is unfortunate."
Trump accused the Democrats of "stealing" staff from the White House and said they were going "nuts".
Schiff's announcement was the clearest sign to date that Democrats will keep probing the President well after Special Counsel Robert Mueller's Russia investigation ends.
It came just a day after he criticised "ridiculous partisan investigations" during his State of the Union address.
"If there is going to be peace and legislation, there cannot be war and investigation," he said during his speech, as he called for unity and bipartisanship.
The line was received with an awkward silence, and even a few laughs, but House Speaker Nancy Pelosi said it would not stop the raft of investigations into Trump's affairs.
"It was a threat; it was an all-out threat," she told reporters after the address. "It's our congressional responsibility, and if we didn't do it, we would be delinquent in that."
Just hours earlier, another probe was announced into alleged corruption by the President's inaugural committee, which the White House claimed was completely unrelated to him.
"The fact that those things have taken place literally have absolutely nothing to do with the President," said press secretary Sarah Sanders. "I think the common thread is a hysteria over the fact that this president became president," Ms Sanders told CNN. "The common thread is that there is so much hatred out there that they will look for anything to try to create and tie problems to this president."
The Russia probe has already led to the arrest of several Trump associates. His former lawyer Michael Cohen pleaded guilty to lying to Congress in his 2017 testimony about when talks around a possible Trump Tower Moscow ended. Trump has distanced himself from Cohen.
Schiff also revealed the committee had voted to release all witness transcripts from its Russia investigation to the Department of Justice and Mueller, after sending Roger Stone's transcript in December at the special counsel's request.
These include interviews with Donald Trump Jr, Jared Kushner and aides Corey Lewandowski, Steve Bannon and Hope Hicks, conducted by the committee during a year-long probe.
Republicans on the panel have argued that the investigation should also look at Hillary Clinton's presidential campaign and whether it colluded with Russia.
But they were overruled by Democrats, who said they had not seen evidence of collusion by the Clinton campaign — and the actions of the President were far more important to the public interest.
Former committee chairman Devin Nunes led several motions that were rejected on Wednesday, including issuing a subpoena to former FBI director James Comey and immediately releasing all transcripts to the public. The Democrats said the interviews contained sensitive information and would be released within months.
"You guys are an embarrassment to yourselves," said Republican Nunes.
It is a major role reversal for the two parties, following the Democrats taking the House majority.
In the last Congress, Schiff made many requests for subpoenas and actions that were rejected by the mostly Republican committee.
The House Ways and Means Oversight Subcommittee is also holding a hearing today regarding obtaining Trump's tax returns.