Red shows TV markets left out if the FCC orders stations in the top 50 to post political ad buys. For a closer view, zoom in; for more information, click on an area.

The Federal Communications Commission is set to vote Friday on a rule that would require broadcast stations to post online their public files, including records of political advertisements. That could provide a crucial source of information about the shadowy groups that can now spend unlimited sums to advocate for or against political candidates without having to register with the Federal Election Commission.

But the proposed rule would also would also exempt 160 of the country's 210 television markets, including some in battleground states likely to be targeted for major political ad blitzes.

Details of the FCC's proposed disclosure requirement, leaked earlier this month, suggest it would apply only to the top 50 markets, shown on the map above in green. And it would apply only to TV stations in those markets affiliated with the four major networks: NBC, ABC, CBS and FOX. That would leave out major stations that dominate their local markets such as Chicago's WGN or Univision in Los Angeles. No Spanish language stations would be included in a year when the Hispanic vote is a key demographic likely to be targeted by both parties.

At issue is the requirement to post the so-called public file online. All radio and television broadcast stations must maintain a public record that contains, among other things, a complete record of a request to purchase broadcast time made by or on behalf of a candidate for public office, or communicates a message relating to any political matter of national importance. That can include, for example, how much a campaign or super PAC is spending on their television advertisements.

Typically, these files are stored in a file cabinet somewhere in the station and are viewed only when a member of the public visits the stations and requests them. As one might expect, few members of the public do.

To increase the accessibility of these public files, open access groups, including the Sunlight Foundation, the Free Press and the Center for Responsive Politics, want broadcasters to post them online. Broadcasters have argued that such requirements would be onerous and expensive, and that there is little demand for the files in the first place. But days before the FCC's vote, they came up with a counter-offer to provide some of the information in return for keeping their ad rates secret.

Though it has been heatedly opposed by broadcasters, the effect of the FCC's proposed rule would be limited if the regulatory agency sticks to the leaked plan. The records to be posted online would likely just be PDFs — making it difficult to consolidate, filter and search the information — even though the files are based on electronic records kept by the station.

And, although roughly two-thirds of the U.S. population lives in the top 50 TV markets, campaign advertising is not constrained to them. Indeed, many large markets, which charge the highest advertising rates, can have relatively little election-related advertising, at least at the presidential level.

For example, the country's two most populous markets, New York and Los Angeles, aren't likely to be contested in the presidential election. Not only are California and New York considered relatively safe states for President Obama, but their advertising rates dwarf that of other markets, making it too expensive to bother with much advertising there.

Yet, large areas of some swing states, like Virginia, Missouri, Wisconsin and Michigan, could see an influx of advertising in markets outside of the top 50. One state that's likely to be a key battleground — Iowa — doesn't have any media markets that would make the reporting cut.

That means data on those important political ad buys would remain offline, accessible only to journalists and citizens willing to pull the paper files — a process that, as one group of Ohio journalism students found this week, can be ardous.