Supreme Court Justice Clarence Thomas’ actions have once again raised questions about unethical conduct of judges after he appeared at a $200-a-plate black-tie fundraising dinner for the Federalist Society, a conservative legal advocacy organization funded in part by the Koch brothers. Justice Thomas was interviewed on-stage at the event Thursday by Judge Diane Sykes of the Seventh Circuit Court of Appeals.

Canon 4C of the Code of Conduct for U.S. Judges prohibits federal judges from using the prestige of their judicial office to fundraise. Specifically, “a judge may not be a speaker, a guest of honor, or featured on the program” of a fundraising event. The code applies to federal judges but not to Supreme Court justices.

Justices Antonin Scalia and Samuel Alito also attended the event, but neither spoke, nor were they otherwise featured in the evening’s program. Chief Justice John Roberts did not attend.

“By headlining this fundraiser, Judge Sykes is clearly in violation of the Code of Conduct for U.S. Judges and Justice Thomas would be as well—if only the Supreme Court was bound by an ethical code,” Rep. Louise Slaughter (D-NY) said in a statement. “The guidelines contained in the Code exist to ensure the public has faith that judicial decision-making is based on the facts and the law, not politics and outside interests. Congress must act to ensure the Supreme Court plays by the same ethical rules as all other federal judges.”

Slaughter is the sponsor of a House bill that would require the Supreme Court to adopt its own ethical code, including all the canons in the existing Code of Conduct for U.S. Judges. Chief Justice John Roberts has said the Supreme Court looks to the code for guidance on ethical questions but has so far avoided making it binding on the nation’s highest court.

“Justice Thomas is among several members of the high court who’ve made a habit of flouting judicial ethics by headlining Federalist Society fundraisers,” said Arn Pearson, vice president for policy and litigation at the nonprofit advocacy organization Common Cause. “He gets away with it because the Court has exempted itself from the Code, but that doesn’t make it right. Our nation’s highest court should not have the lowest ethical standards.”

“Here we have the spectacle of two judges helping the Federalist Society raise funds,” added Alliance for Justice President Nan Aron. “Yet while Judge Sykes is violating a binding code of conduct, Justice Thomas is not, because the Supreme Court refuses to be bound by any formal, written code of ethics. Congress needs to change that.”

Rep. Slaughter, along with Common Cause and the Alliance for Justice, has asked Chief Justice John Roberts to address Thomas’ conduct. Thomas’ wife, Virginia Thomas, has deep ties to conservative activists and organizations that often lobby on matters that end up before the Supreme Court. Those ties include organizations linked to billionaire activists the Koch brothers, who have funneled millions into anti-reproductive health and justice causes in addition to groups like the Federalist Society.

Rep. Slaughter and the organizations also filed a formal ethics complaint with the Seventh Circuit Court of Appeals against Judge Sykes.

“We hope these ethics complaints move the courts to take their neutrality seriously and spur Congress to make the Code of Conduct binding on the Supreme Court,” Pearson said. “Judges undermine the integrity of our legal system when they lend their prestige to fundraising efforts, particularly by groups that have an ideological agenda or have proceedings before the courts. It is time to stop the growing politicization of American courts.”