President Donald Trump is ramping up his criticism of Robert Mueller, saying that the special counsel investigating Russian meddling in 2016 election, would cross the line by looking into his finances, The Hill reports.

Mueller is now reportedly looking at transactions involving Trump’s businesses and those of his associates. The development is sure to exacerbate Trump’s frustration with the Russia investigation, an investigation that has dogged his presidency and forced him and his closest aides to hire legal representation.

Among other things, Mueller is said to be probing potential obstruction of justice by Trump, following the president’s decision to fire FBI director James Comey at the height of the bureau’s investigation into Russian meddling — an event that precipitated Mueller’s appointment.

In recent weeks, Mueller has been building an all-star team of more than a dozen lawyers for the investigation; some of those appointees have come under scrutiny for donating money to Democratic political candidates.

All the while, the president has deemed the investigation a “witch hunt.”

The former FBI director who has a reputation in Washington as a straight shooter, was appointed special counsel by the Justice Department in May. In that role, he has broad authority to investigate Russia’s meddling as well as any ties between associates of Trump’s campaign and Moscow.

Legal experts say it’s only natural that Mueller’s investigation would look at aspects of Trump’s businesses.

“I am totally not surprised that Mueller is following any leads,” said Steven Cash, an attorney at Day Pitney’s Washington, D.C., office. “That’s the way all investigations are conducted, particularly into complex relations of business people.”

But some Republicans say they agree with Trump that Mueller is overreaching.

“Mueller crosses ‘red line’ into potentially all of Trump’s billion$ in transactions. We now face a partisan war of investigative attrition,” Representative Steve King tweeted Thursday.

The renewed focus on Mueller was sparked by an interview with The New York Times where Trump said it would be a “violation” for Mueller to examine his or his family’s financial dealings, though he declined to say how he would react if Mueller did so.

Trump did, however, hint that he had damaging information on Mueller, noting that he interviewed him in May as a potential candidate to replace Comey.

“The next day, he is appointed special counsel. I said, what the hell is this all about? Talk about conflicts? But he was interviewing for the job,” Trump told the Times. “There were many other conflicts that I haven’t said, but I will at some point.”

The president at one point added, “I have done nothing wrong. A special counsel should never have been appointed in this case.”

The comments have again raised the specter that Trump could fire Mueller, an action that Republicans have repeatedly warned him against.

Senator Marco Rubio, who sits on the Senate Intelligence Committee, told reporters Thursday that “it would be a mistake to fire Bob Mueller,” calling him “a highly credible individual who will do a good job.”

But Jay Sekulow of the American Center for Law & Justice, a member of the president’s legal team, floated the possibility that Trump could fire the special counsel last month if he deems that the probe is not being conducted properly.

“The president has authority to take action,” Sekulow told ABC News. “Whether he would do it is ultimately a decision the president makes.”

During the Times interview, Trump also took aim at longtime ally Jeff Sessions, indicating he would not have appointed him as attorney general if he knew he would recuse himself from the Russia probe.

Similarly, he jabbed at Deputy Attorney General Rod Rosenstein, who appointed Mueller, suggesting he’s a Democrat because he is from Baltimore. Rosenstein, a Republican, was born in Philadelphia and worked in Baltimore as a prosecutor.

Some say that Trump’s public comments about Mueller, including those in previous interviews, could ultimately be used against him as evidence.

“Mueller like any good prosecutor will be looking at tweets, interviews, the Lester Holt interview, other public and private comments as potential admissions that are indicators of the president’s true intent,” said Ron Hosko, who served as assistant director of the FBI’s criminal investigative division under Mueller.

“I think [Trump] is really on slippery terrain when he makes these public comments that … if charges were to come, could be potentially used as evidence of his intent.”

“Depending how the rest of the case lines up, it could enhance his risk,” Hosko added.

Trump’s attacks on Justice Department officials threaten to create a wedge between the president and Republican lawmakers, many of whom have expressed high regards for Mueller and used to work alongside Sessions.

A GOP consultant said Trump’s comments about Mueller are likely to be met with concern by Republicans publicly, but noted that they are just the latest in a string of controversial actions by the president that Republicans have had to grapple with.

Likening the situation to successive lice outbreaks, the source predicted that Republicans would continue to stick by Trump.

“They’re tainted with him, they’ve stuck with him through pretty unimaginable things,” the source said. “You’re already scratching your head. One more bug is not going to make the difference.”

Despite Trump’s statements, Mueller’s investigation shows no sign of slowing down.

According to Bloomberg, investigators are examining Russian purchases of Trump properties, the 2013 Miss Universe pageant in Moscow, and Trump’s role in a New York City development involving Russians, among other things.

“What I have seen and read and heard in the last month or so is that Mueller seems to have been hiring attorneys with substantial histories in complex white collar crime, financial crimes,” Hosko said.

“The number of people that he has hired starts to suggest that this will be an expansive — and if it’s expansive, it will be lengthy.”