Senate Majority Leader John Thune, R-S.D., says the criminal investigation into Federal Reserve Chair Jerome Powell must not do anything to harm the central bank's independence.

"I haven't seen the case or whatever the allegations or charges are, but I'd say they better be real and they better be serious," Thune told reporters Monday.

"It needs to be resolved quickly because the Fed's role and the Fed's independence in shaping monetary policy in the country is something we need to ensure proceeds without political interference," he added.

The Department of Justice subpoenaed the Federal Reserve on Friday in an investigation tied to Powell's June Senate testimony over ballooning renovation costs at the Fed's headquarters.

Powell confirmed the subpoena Sunday, saying the inquiry is part of a pressure campaign fueled by the administration's frustration with the Fed's interest rate policy.

"This is about whether the Fed will be able to continue to set interest rates based on evidence and economic conditions — or whether, instead, monetary policy will be directed by political pressure or intimidation," Powell said in a video statement.

President Donald Trump said Sunday that he had no knowledge of DOJ's investigation into the Federal Reserve, which White House Press Secretary Karoline Leavitt reiterated Monday.

"Look, the president has every right to criticize the Fed chair," Leavitt said. "He has a First Amendment right, just like all of you do."

"And one thing for sure, the president's made it quite clear, is Jerome Powell is bad at his job," Leavitt added.

"As for whether or not Jerome Powell is a criminal, that's an answer the Department of Justice is going to have to find out, and it looks like they intend to find that out," she continued.

Sen. Thom Tillis, R-N.C., is threatening to block all of Trump's Federal Reserve chair nominees amid the investigation.

In a Sunday night statement, Tillis, who is not seeking reelection this year, warned that Justice's investigation of Powell threatens the Fed's independence.

"If there were any remaining doubt whether advisers within the Trump Administration are actively pushing to end the independence of the Federal Reserve, there should now be none. It is now the independence and credibility of the Department of Justice that are in question," Tillis said.

"I will oppose the confirmation of any nominee for the Fed — including the upcoming Fed Chair vacancy — until this legal matter is fully resolved," Tillis added.

He told reporters the investigation into Powell could make it challenging to confirm Trump's nominees to the Fed.