Handing open-government advocates a partial victory in a better than decade-long battle, the Federal Communications Commission voted Friday to require major network affiliates in the top 50 TV markets to post information about their political ads online.

The move could provide the public with crucial information on who's behind the ads purchased by nonprofit groups that, under new campaign finance laws, can spend unlimited amounts of money advocating for or against candidates without registering with the Federal Election Commission. But the rule will leave out many TV markets in presidential battleground states as our map shows.

Broadcasters had fiercely opposed the move as  "burdensome," but FCC Chairman Julius Genachowski offered  evidence for how burdensome it is for citizens to obtain the information through paper files. When the FCC tried to obtain information from Baltimore stations, the agency found it took 61 hours to obtain the information from eight local stations, and they were presented with a copying bill of $1,700. 

"The question for us is whether in the 21st century available for public inspection means stuck in office filing cabinets or available online," Genachowski said. 

Commissioner Mignon Clyburn, the daughter of veteran Rep. Jim Clyburn, D-S.C., and a swing vote in the case, strongly backed the move. "The public is our greatest watchdog," she said.

Dennis Wharton of the National Association of Broadcasters told reporters afterward that the trade group will be reviewing its legal options. "We will be seeking guidance from our board of directors," he said. Corie Wright, an attorney for the Free Press, a member of the coalition that has been pressing for online posting of the files, praised the FCC for "giving the public meaningful access to the public files," in a statement.

About three dozen people — including about 10 news reporters — were in the room for the vote. One lawyer for an evangelical radio station, who did not want to be quoted, says he's concerned that what starts in TV will "bleed into radio" and worried that this will mean someone in Maine being able to dig into files of a station in Alabama. Under the in-person method of file-checking, broadcasters know who is going through their files. 

The Sunlight Foundation, in partnership with the Free Press and other open-government groups, is sponsoring an effort to put information from the political ad files that will be posted — and those that will not — in a publicly available online database that users will be able to sort, search and filter. Interested in helping? Email us here or check out these helpful tips from Free Press.