Democrats on the Senate Judiciary Committee called on U.S. Supreme Court Chief Justice John Roberts on Monday to open an investigation into the undisclosed acceptance of luxury trips taken by Justice Clarence Thomas and his wife that were paid for by a Republican megadonor.

The letter said the committee plans to hold a hearing in coming days regarding the “need to restore confidence in the Supreme Court’s ethical standards.” And if the Supreme Court does not deal with the issue on its own, the committee will consider voting on legislation. Such a measure would also need support from the Republican-led House to become law.

“But you do not need to wait for Congress to act to undertake your own investigation into the reported conduct and to ensure that it cannot happen again,” the 11 Democratic senators wrote to Roberts. “We urge you to do so.”

The nonprofit investigative journalism organization ProPublica reported Thursday that Thomas, who has been a justice for more than 31 years, has for more than two decades accepted luxury trips from Republican donor Harlan Crow nearly every year.

Thomas, 74, and his wife, Virginia, have traveled on Crow’s yacht and private jet as well as stayed at his private resort in New York’s Adirondack Mountains, ProPublica reported. A 2019 trip to Indonesia the story detailed could have cost more than $500,000 had Thomas chartered the plane and yacht himself.

The Democratic senators said Thomas’ acceptance of favors from Crow was known more than a decade ago. They noted that senators then had urged the Supreme Court court to adopt a resolution stating that the justices abide by the ethics rules that the rest of the federal judiciary follows.