WASHINGTON  Senator Bernard Sanders of Vermont said on Wednesday that he would try to block the Senate from confirming Ben S. Bernanke to a second term as chairman of the Federal Reserve.

The move is unlikely to derail Mr. Bernanke’s reappointment, but it could slow the confirmation process and give the Fed’s critics additional opportunity to press their case. As a practical matter, it means Senate Democratic leaders will have to line up 60 votes in favor of Mr. Bernanke rather than a simple majority at a time when the Federal Reserve is under increasing populist attacks from lawmakers on both the right and the left.

Mr. Bernanke will testify on Thursday at his confirmation hearing before the Senate banking committee. He is expected to face criticism for not doing more to prevent the financial crisis, and calls by some lawmakers for a sharply reduced regulatory role in the future.