• Sen. Ron Wyden asked GOP megadonor Harlan Crow for a complete list of gifts to Supreme Court Justice Clarence Thomas.
• The Oregon Democrat also requested evidence that the billionaire Texas real estate developer complied with federal tax law in connection with the long-undisclosed largesse to Thomas.
• Wyden's letter comes amid criticism of Thomas following a ProPublica report that the CEO of Crow Holdings has treated the conservative justice to luxurious trips.

Sen. Ron Wyden, D-Ore., on Monday asked GOP megadonor Harlan Crow for a complete list of gifts to Supreme Court Justice Clarence Thomas and evidence that the billionaire real estate developer complied with federal tax law in connection with the long-undisclosed largesse to Thomas.

"This unprecedented arrangement between a wealthy benefactor and a Supreme Court justice raises serious concerns related to federal tax and ethics laws," Wyden, who heads the Senate Finance Committee, wrote in a six-page letter to Crow.

Wyden's letter was sent as Thomas and the Supreme Court itself face criticism following an April 6 report by ProPublica that the chairman of Crow Holdings for more than two decades has treated the conservative justice to luxurious trips worth at least hundreds of thousands of dollars.

ProPublica also reported on April 13 that a Crow company in 2014 purchased three properties in Savannah, Georgia, from Thomas and his family, including a home where the justice's mother has lived rent-free for more than a decade.

The gifted trips to Thomas and his wife, Ginni, were to places such as Indonesia, New Zealand and Greece, with travel on Crow's private jet and 162-foot superyacht Michaela Rose.

Thomas had not disclosed any of the gifts from Crow, or the property purchases by him, until they were revealed by ProPublica.

"The secrecy surrounding your dealings with Justice Thomas is simply unacceptable," Wyden wrote in his letter to Crow.

"The American public deserves a full accounting of the full extent of your largesse towards Justice Thomas, including whether these gifts complied with all relevant federal tax and ethics laws," he wrote.

The letter asks for a list of all flights Thomas took on any of Crow's jets, as well as details of those trips. Wyden requested similar details about the justice's trips on the Michaela Rose and information about the Georgia property purchases.

He concluded by writing, "Please list any additional gifts or payments with a value in excess of $1,000 made to Justice Thomas or members of his family since he was sworn into the Supreme Court that
would not be captured by" the prior questions.

Wyden's letter noted that federal tax law requires the giver of a gift to pay any applicable tax.

"The IRS has long made clear the gift tax applies to the transferor of a gift, including in cases where
the transferor provides for the 'use of property' without expecting to receive something of at
least equal value in return," Wyden wrote.

In addition to asking Crow for evidence related to the possibility of gift taxes being owed by the business, Wyden asked whether Crow claimed business deductions or depreciation for his plane and yacht related to the trips by Thomas.

Wyden is the ranking Senate Democrat on Congress' Joint Committee on Taxation.

A spokesman for Crow did not immediately respond to a request for comment from CNBC about Wyden's letter. Thomas did not immediately respond to a request for comment sent to the Supreme Court's media affairs office.

Last week, Sen. Dick Durbin, the Illinois Democrat who is chairman of the Senate Judiciary Committee, invited Supreme Court Chief Justice John Roberts to testify about ethics reform of the high court.

Durbin's letter to Roberts noted that "there has been a steady stream of revelations regarding Justices falling short of the ethical standards expected of other federal judges."

Roberts has yet to reply to that invitation, Durbin noted over the weekend.