Arthur Hellman, an ethics expert at the University of Pittsburgh law school, said O'Connor should consider stopping her participation in court cases if she "wants to engage in this level of political or politically related activity."
Partisan-tinged questioning of conduct by high court justices has grown.
Liberal interest groups have faulted Justices Antonin Scalia and Clarence Thomas for speaking at a private dinner hosted by Charles Koch, one of two energy-company-owning brothers who, liberal groups say, have too much sway on policymakers.
Some liberals have called on Thomas to sit out the expected high court fight over the health care law because of his wife's public criticism of the law. Some conservatives say Justice Elena Kagan should not take part in the health care case because of her work in the Obama administration before joining the court.
O'Connor has traveled the country since her retirement in 2006 to criticize costly election campaigns for state judges, promote enhanced civics education for schoolchildren and advocate for Alzheimer's research. Her husband, John, died in 2009 of complications from Alzheimer's disease.
Her primary focus has been on judicial independence, which she believes is harmed by electing judges.
At the same time, she has heard cases on appeal since her retirement. It is not uncommon for retired justices to sit with federal appeals courts from time to time. Justice David Souter, who left the court in 2009, has heard cases with the Boston-based court.
Through the end of March, O'Connor had written two appellate decisions and joined the majority in a half-dozen others this year. None of the cases involved judicial elections or the fate of the Alaska bay.
The continuing judicial work allows O'Connor, who earns $213,900, to receive salary increases that are tied to inflation. Judges who stop hearing cases receive a pension equal to their final annual salary as a full-time judge, but are excluded from subsequent cost-of-living increases.
Last week, O'Connor was the host of a Supreme Court reception "to celebrate the economic, cultural and ecological values of Alaska's Bristol Bay Watershed."
Opponents of the proposed huge mine near the bay fear it will devastate the world's largest wild sockeye salmon fishery. The Environmental Protection Agency recently said it would study potential effects from the mine. EPA Administrator Lisa Jackson attended and briefly spoke at the reception.
O'Connor declined to answer questions for this story. But the head of the Wild Salmon Center, lead sponsor of the event, said O'Connor's participation came about because of their friendship and her love of fly fishing.
Guido Rahr, the center's president, said his group hasn't taken a position on the mine and that the speakers were careful to "make sure we were respecting the location" of the reception at the court.
Rahr said participants mainly "ate yummy salmon treats" and looked at National Geographic photos of the bay.
But one speaker was a former Alaska state Senate president, Rick Halford, who told reporters the next day that the proposal was a "very, very dangerous kind of mine."
Supporters of the project made their own visit to Washington a couple of weeks earlier. They were not received at the high court.
Hellman, the Pittsburgh ethics expert, said he finds the court reception particularly troubling because "we're talking about political activity. It's a lobbying effort and she is lending her considerable prestige to that effort."