A little-known conservative non-profit is bankrolling the legal representation of a Roger Stone associate as part of an effort to challenge Special Counsel Robert Mueller’s investigation.

Attorney Paul Kamenar, who represents longtime Stone associate Andrew Miller, told The Daily Beast he is taking a reduced rate because he believes the special counsel probe is unconstitutional. He said the National Legal and Policy Center (NLPC) is helping fund his work because they share his Constitutionality concerns.

This is all part of a effort to have a judge find that Mueller’s investigation is against the law.

Peter Flaherty, the chairman of the board of directors for the National Legal and Policy Center, told The Daily Beast that Miller is his group’s only Mueller-related client.

“We needed one client for this project and we have him,” he said. “We’re committed to paying all the costs associated with this Constitutional challenge.”

Miller has been close with Roger Stone for years. Kristin Davis, the so-called “Manhattan Madam” who testified before Mueller’s grand jury on Friday, said Miller handled Stone’s schedule during the presidential campaign, according to NBC News.

On Friday, Miller defied a subpoena and refused to appear before Mueller’s grand jury. His lawyers argued that Mueller’s subpoena was invalid because the special counsel did not have the authority under the Constitution to issue it.

Judge Beryl Howell rejected the argument and subsequently held Miller in contempt of court. But she stayed the contempt order since his attorney plans to appeal her ruling to the Court of Appeals for the District of Columbia. As a result, Miller is not incarcerated.

“This is an opportunity for us to bring this challenge,” Kamenar told The Daily Beast.

He noted that lawyers for the Russian company Concord Management—alleged to have participated in Russian social media manipulation efforts—have made a similar argument. But their argument will move through court much more slowly than Kamenar and Miller’s because legal disputes about grand jury proceedings are generally handled expeditiously.

Kamenar is arguing that Mueller’s appointment violates the Constitution’s appointments clause because only the attorney general—in this case, Jeff Sessions—has the authority to appoint a Special Counsel. He also argues that there’s no law on the books giving the Justice Department the power to appoint a Special Counsel, and that the Special Counsel should have been required to go through Senate confirmation because he is as powerful (if not more so) than U.S. Attorneys.

Sessions recused himself from the Russia investigation in March of 2017, and the Justice Department appointed Mueller, a former FBI director, as special counsel in May.