Michael Copps, a longtime Democrat on the Federal Communications Commission, says he’s retiring by year’s end.

Copps, a vocal critic of media consolidation, has been a commissioner since May 2001. He did not give a reason in his public resignation letter Tuesday. He says he still plans to speak about broadband expansion and other issues as a private citizen.

Last year, Copps was the only commissioner to vote against Comcast Corp.’s takeover of NBCUniversal, saying the deal "confers too much power in one company’s hands."

His departure would leave two Democrats and one Republican on the FCC, with two vacancies. President Barack Obama has already nominated Jessica Rosenworcel, a former legal adviser to Copps, and Ajit Pai, another former FCC staffer, to fill the seats.