"We do not believe the committee has the authority to investigate Mr. Crow's personal friendship with Justice Clarence Thomas," read the letter from Crow's lawyers.

"Most importantly, Congress does not have the constitutional power to impose ethics rules and standards on the Supreme Court."

The committee's chairman, Senator Dick Durbin, said Crow's lawyers did not provide credible justification for refusing to respond to the panel's questions.

He said the committee will respond to the letter and continue to seek answers to members' questions.

"Mr. Crow’s letter relies on a separation of powers defense when Mr. Crow does not work, and has never worked, for the Supreme Court," Durbin said in a statement.

"Harlan Crow believes the secrecy of his lavish gifts to Justice Thomas is more important than the reputation of the highest court of law in this land. He is wrong."

Democrats on the Senate Judiciary Committee on May 9 asked Crow to detail gifts to Thomas. They said reporting has identified multiple instances in which Crow or entities he owns or controls have made payments, bought real estate, or provided gifts, travel, or other items of value to Thomas.

"Many of these gifts, transactions, and items of value had not been previously disclosed by Justice Thomas," the Democrats' letter said.

A representative for the Supreme Court did not immediately respond to a request for comment.

The Democratic-led Senate panel has explored the possibility of pursuing legislation to impose ethical standards on the Supreme Court amid revelations about luxury trips and real estate transactions by conservative justices, but Republican members voiced stern opposition.

The news outlet ProPublica has detailed ties between Thomas, a conservative who is the court's longest-tenured member, and wealthy Republican donor Crow, including real estate purchases and luxury travel paid for by the Dallas businessman.