FCC Commissioner Michael Copps formally announced his retirement on Tuesday just days before a key Senate vote for two nominees to the agency.

The departure of the 71-year-old Democrat, which had been expected, takes effect Jan. 1, unless the Senate confirms his replacement or adjourns before then.

The Senate Commerce Committee is set to vote on two Commissioner nominees on Thursday: Jessic Rosenworcel and Ajit Pai. Both are attorneys who have previously worked at the FCC and have received broad bipartisan support for their appointments.

The nominees could run into trouble, however, if Sen. Charles Grassley follows through on his threat last week to hold up a floor vote on the nominees. The Iowa Republican wants to block the candidates because of the FCC's continued refusal to provide documents about its interactions with LightSquared and its primary backer, Harbinger Capital Partners.

Grassley, who is the ranking member of the Senate Judiciary Committee, wants information on why the FCC granted LightSquared a waiver for its hybrid satellite-terrestrial LTE network in January despite concerns about GPS interference expressed by other federal agencies, including the Defense Department and NTIA.

There have been allegations that the waiver was linked to campaign contributions made to the Obama administration by Harbinger Capital Partners. Harbinger and LightSquared have denied any impropriety.

The FCC recently released a flood of documents about LightSquared in response to Freedom of Information Act requests, but the files have not been enough to satisfy Grassley.

The FCC has been operating with just four of its usual five commissioners since Republican Commissioner Meredith Baker Atwell left in June to take a lobbying job at Comcast.

Copps has served on the FCC since 2001 and was sworn in for his second term at the agency in 2006. The new term ran through June 30, 2010. Once their terms end, FCC commissioners can serve until they get a replacement or the congressional session ends.

During his 11-year stay at the FCC, Copps was a vocal critic of what he called "excessive" consolidation in the media and telecommunications space. He was opposed to the FCC's approval earlier this year of the merger between Comcast and NBCUniversal and said he "welcomed" AT&T's decision to withdraw its application to acquire T-Mobile USA.