WASHINGTON, Jan. 20 (UPI) -- A U.S. advocacy group trying to overturn the Supreme Court's controversial Citizens United opinion says two justices have a conflict of interest in the case.

The evidence cited by Common Cause is an invitation sent by conservative billionaire Charles Koch, a major political donor, to a political retreat, The New York Times reported Thursday. The invitation says justices Antonin Scalia and Clarence Thomas are among the "notable leaders" who have participated in past conferences.


Common Cause, a campaign finance watchdog, asked the Justice Department to investigate whether the two justices had a conflict of interest when they participated in the Citizens United decision. In its petition, filed Wednesday, the group also pointed to the role played by Thomas' wife, Virginia, in organizing a group opposed to President Barack Obama.

Supreme Court justices generally decide when to recuse themselves for conflict of interest. Scalia was involved in deciding a case involving Vice President Dick Cheney in 2004 even though he and Cheney had been on a hunting trip together.

"It's a steep uphill climb for Common Cause, but not an insurmountable one," said Steven Gillers, a law professor at New York University.