"If I was the city, I'd arrest him every time for impersonating a police officer," said John McNesby, president of the Fraternal Order of Police Lodge 5.
Images of a uniform-wearing Lewis being arrested at Occupy Wall Street last year went viral on the Internet.
When Time magazine named "the Protester" as the 2011 Person of the Year, an entire page was devoted to a portrait photograph of Lewis.
Lewis, a 24-year police veteran who retired eight years ago, said that he had been notified by FOP that the uniform controversy would come up at an April 2 meeting.
The FOP had sent him a letter in November saying the matter had been referred to the union's grievance committee.
"They're threatening me, hoping they'll take away a white police captain's face from the Occupy movement," said Lewis, who was in uniform Monday across the street from City Hall to protest efforts to stop him.
McNesby said that he did not expect any action at next month's meeting, but that ultimately the grievance committee could recommend actions ranging from a reprimand to removal from the union.
If Lewis is expelled, he would lose benefits including his life insurance and access to legal assistance, McNesby said.
Lewis, 60, called the FOP action a "farce" and said he would continue to protest corporate abuses.
Contact Robert Moran at 215-854-5983 or firstname.lastname@example.org, or follow on Twitter @RobertMoran215.