Chief Justice John Roberts will not testify at an upcoming hearing of the Senate Judiciary Committee focused on Supreme Court ethics.

The committee’s chairman, Senator Dick Durbin (D., Ill.), invited Roberts to testify last week, saying “the time has come for a new public conversation on ways to restore confidence in the Court’s ethical standards.” The move came amid efforts by Democrats to force the high court to adopt ethics rules and calls to investigate Justice Clarence Thomas for his financial dealings with billionaire Harlan Crow.

“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,” wrote Roberts in response.

The chief justice noted that the Supreme Court Library has only two recorded instances of it happening in the past: Chief Justice Taft in 1921 and Chief Justice Hughes in 1935. Both hearings involved routine matters of judicial administration, Roberts wrote, adding that his predecessor, Chief Justice Rehnquist, appeared before House committees twice on similarly mundane topics.

“Neither Chief Justice Burger nor Chief Justice Warren nor Chief Justice Vinson ever appeared before a Congressional Committee,” explained Roberts.

“Congressional testimony from the head of the Executive Branch is likewise infrequent,” he added.

Asked last week if he was considering a subpoena should Roberts decline, Durbin demurred, appearing to allude to the absence of Senator Dianne Feinstein (D-Calif.). “It takes a majority. I don’t have a majority,” Durbin told reporters.

The Illinois senator was also asked this weekend why he didn’t invite Justice Clarence Thomas directly.

“I think I know what would happen to that invitation. It would be ignored,” Durbin said.