Chief Justice John Roberts on Tuesday declined an invitation from a high-ranking Democratic senator to testify at a congressional hearing on ethics rules for members of the Supreme Court.

In a letter to Senate Judiciary Committee Chairman Dick Durbin, D-Ill., Roberts suggested that his participation could pose a threat to judicial independence.

“Testimony before the Senate Judiciary Committee by the Chief Justice of the United States is exceedingly rare, as one might expect in light of separation of powers concerns and the importance of preserving judicial independence,” Roberts wrote.