President Donald Trump has spent months criticizing Federal Reserve Chairman Jerome Powell over interest rates, repeatedly arguing they should be lower. Now the Justice Department has opened a criminal investigation tied to Powell’s testimony on a costly Fed renovation project, prompting swift criticism from economists and former Fed chairs.

On Capitol Hill, a small but vocal group of Republicans has warned that the investigation risks politicizing the central bank at a sensitive moment for monetary policy. The sharpest response has come from Sen. Thom Tillis (R-NC), who is not running for reelection. Tillis recently said he would oppose any Fed nominee put forward by the White House until the matter involving Powell is resolved.