(Bloomberg) — President Donald Trump said he would announce his nomination to replace Federal Reserve Chair Jerome Powell within "the next few weeks" despite the backlash over a Justice Department probe into the remodeling of the central bank's headquarters that has threatened his ability to secure a confirmation.

Trump was asked about his plans during a factory tour in Michigan, and indicated he planned to push ahead despite concern expressed by key Republican lawmakers. The president also criticized Senator Thom Tillis, a retiring North Carolina Republican and swing vote on the Senate Banking Committee, who has said he would block nominations to the central bank until the investigation was resolved.

"That's why Thom's not going to be a senator any longer," Trump said.

Earlier Tuesday, Trump offered at least a tacit defense of the probe — which Powell and critics have called an effort to influence policy — criticizing the chair over cost overruns during the construction project."He's billions of dollars over budget, so, either he's incompetent or he's crooked," Trump said at the White House. "I don't know what he is, but he certainly doesn't do a very good job."

The probe marks a dramatic escalation of the Trump administration's attacks on the Fed and raises new questions about the institution's independence. Trump has repeatedly toyed with trying to fire Powell before his term as chair ends in May, and is currently weighing who to nominate as his successor.

But the move to open a criminal investigation, which was disclosed over the weekend, prompted blowback from Republican senators. That development threatens to derail the president's eventual nominee as the next chair, and with it, his effort to exert greater control over the Fed.

A Fed spokesperson declined to comment on Trump's remarks Tuesday.

Powell said on Sunday that the Justice Department had served the Fed with grand jury subpoenas, stemming from a probe into the renovation project and Powell's testimony to Congress about it. 

In addition to Tillis, GOP Senators Lisa Murkowski and Kevin Cramer have also criticized the move.

Three former Fed chairs and four former Treasury secretaries representing Republican and Democratic administrations decried the investigation in a joint statement, saying "it has no place in the United States whose greatest strength is the rule of law, which is at the foundation of our economic success."

Trump had previously seemed to distance himself from the investigation, telling NBC News he didn't know anything about the subpoenas. Trump is a longtime critic of Powell and has repeatedly demanded the Fed lower rates. He has also said that he won't pick a new chair unless they commit to lowering rates, a litmus test that further shakes the pillars of the Fed's independence. 

Powell, in a statement Sunday, called the renovation investigation "pretexts" for a broader pressure campaign about rates. 

"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," he said.