Chief Justice John G. Roberts Jr. has declined an invitation to testify at a Senate Judiciary Committee hearing next week on ethics rules for the Supreme Court.

Chair Richard J. Durbin, D-Ill., had invited Roberts to testify at the May 2 hearing, as congressional Democrats respond to reports that Justice Clarence Thomas did not disclose luxury trips and a real estate transaction with a billionaire GOP donor.

Roberts responded in a one-page letter Tuesday that said he “must respectfully decline your invitation.”

Roberts wrote that it is “exceedingly rare” for justices to testify before Congress about anything other than annual appropriations, “as one might expect in light of separation of powers concerns and the importance of preserving judicial independence.”

The letter included a copy of the current “Statement of Ethics Principles and Practices to which all current Members of the Supreme Court subscribe.”

In a statement released after the letter, Durbin said that Thomas’ actions had demonstrated that Supreme Court ethics reform “must happen” whether Roberts is involved or not.

“The actions of one Justice, including trips on yachts and private jets, were not reported to the public. That same Justice failed to disclose the sale of properties he partly owned to a party with interests before the Supreme Court,” Durbin said.

“It is time for Congress to accept its responsibility to establish an enforceable code of ethics for the Supreme Court, the only agency of our government without it,” the Illinois Democrat said.

Durbin had invited Roberts, or any justice he selected, to testify about “the ethical rules that govern the Justices of the Supreme Court and potential reforms to those rules,” in a letter last week.

In an appearance on NBC’s “Meet the Press” Sunday, Durbin said he would not be able to subpoena Roberts or Thomas if they declined to testify because Democrats do not currently have a majority of the committee.

The debate over Supreme Court ethics, which has simmered for years, came to a boil following reports in ProPublica earlier this month that Thomas received trips, vacations and resort stays from Texas billionaire Harlan Crow, as well as selling property to the donor.

Before the stories broke, the Judicial Conference announced a change to its reporting rules, narrowing the exception for “personal hospitality.”

In a letter released Sunday by Sen. Sheldon Whitehouse, D-R.I., Judge Roslynn R. Mauskopf, director of the Administrative Office of U.S. Courts, wrote that the concerns about Thomas’ disclosures had been referred to the Judicial Conference Committee on Financial Disclosure, which handles ethics complaints.

Democrats have called the reports on Thomas the latest sign of the need for ethics reforms at the court, which does not have binding ethics or recusal standards. They have pointed to legislation from Sen. Sheldon Whitehouse, D-R.I., and Rep. Hank Johnson, D-Ga., that did not advance through either chamber last Congress.

Republicans in Congress have backed Thomas and accused Democrats of targeting a prominent conservative. With Republicans in control of the House and a narrowly divided Senate, any Supreme Court legislation would require a bipartisan majority.