Harlan Crow says the Constitution bars investigation into his relationship with Clarence Thomas

Lawyers for Harlan Crow, the billionaire who made headlines for his close relationship with Supreme Court Justice Clarence Thomas, are arguing its unconstitutional for Congress to investigate their connection.

“After careful consideration, we do not believe the Committee has the authority to investigate Mr Crow’s personal friendship with Justice Clarence Thomas,” Mr Crow's legal team said in a letter sent to the Senate Judiciary Committee on Monday, according to Bloomberg.

The attorneys went on to claim that Congress is "not authorised to conduct an ethics investigation of a Supreme Court Justice," and that trying to enforce an ethical ruling "would be unconstitutional."

Democratic Senator Dick Durbin disagreed with the attorney's summation.

“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 said in a statement. "He is wrong."

The Senate Judiciary Committee asked for Mr Crow to turn over information about the gifts and travel he gave to Mr Thomas.

If the Democrats decide to proceed with the investigation, they could attempt to subpoena Mr Crow, a move Mr Durbin has been reluctant to make up to this point, according to The New York Times.

The relationship between Mr Crow and Mr Thomas was detailed in a report by ProPublica, which found the billionaire had been providing the Supreme Court Justice luxury travel for years. It also found that Mr Crow had purchased real estate from Mr Thomas and paid for the private school tuition of one of the Supreme Court Justice's relatives.

None of the arrangements were reported on Mr Thomas's financial disclosures.

Mr Thomas has since claimed he did not believe he needed to report the travel because of an exemption for personal hospitality from friends.

Other members of the Supreme Court have argued that while they are not legally bound by disclosure rules that apply to the rest of the federal judiciary, they do abide by them voluntarily.

The Senate committee invited Chief Justice John Roberts to discuss ethics requirements for the court, but he declined the invitation.

Mr Crow has insisted that he has never discussed Supreme Court cases with Mr Thomas during their travel.

“I have never, nor would I ever, think about talking about matters that relate to the judiciary with Justice Clarence Thomas,” he told The Atlantic.

However, he later added that "it’s not like we haven't talked about work-related issues."

“It’s not realistic for two people to be friends and not talk about their jobs from time to time,” he told the magazine, but insisted they did not talk about individual cases. “From my point of view, that is off limits. He and I don’t go there.”