The Federal Communications Commission should not back down from requiring all broadcast stations to post political ad purchase disclosures online in time for next year's congressional midterm elections, the Sunlight Foundation and a coalition of public interest groups urged in a legal brief filed Monday. The National Association of Broadcasters, meanwhile, appeared to soften its longstanding opposition to online posting of the information.

Sunlight and its allies urged the commission to expand the reach of the current online file -- which compiles information on political ads purchased at about one-tenth of the nation's broadcast stations -- and to push for fully electronic filing, which would make much of the information easier to analyze.

"Online public filings have achieved many of the commission's public interest goals," Sunlight and members of the Public Interest Public Airwaves Coalition wrote in their brief. "At the same time, there are ways the process can and should be improved."

The National Association of Broadcasters, which spent years battling online disclosure as an onerous burden, acknowledged in its brief that the experience of the limited number of stations that did have to post ad files online was "uneventful" and did not oppose expanding the system, but urged the FCC not to make it permanent until more stations could try it out. The NAB also argued that cable and satellite TV providers should be required to post their political ad files online.

Information in those files has become increasingly important because of the proliferation of new political groups, thinly disguised as social welfare organizations, that do not have to register with the Federal Election Commission. Following the Supreme Court's 2010 decision in Citizens United, such groups have become a favorite conduit for campaign contributions. Often the only clues to their agendas or political leanings lie in the paperwork they are required to file at TV stations where they purchase ads.

Last year, the FCC won a court decision forcing broadcast stations in the nation's top 50 TV markets and affiliated with the nation's top four television networks to post political ad files online. The rest remain in filing cabinets, accessible only to those members of the public who manage to make their way to the stations during business hours to see them. The commission proposed expanding its online filing order to all of the nation's broadcast stations in 2014, but said it would first review the pilot project. That review is now underway.

Expanding the filing requirements could have a big impact on transparency in next year's elections. Of the 10 races that veteran political observers believe will determine control of the Senate in 2014, more than half will take place in states that have no online ad disclosure under the current FCC order. Alaska, Arkansas, Louisiana, Iowa, Montana and South Dakota have no stations required to post political ad files online. And the only online posting for the New Hampshire race will take place in the Boston market.

Despite the limited nature of the current order, journalists for Sunlight and other organizations have made good use of the online files to gain insights into political campaigns and to identify the financial and political backers hiding behind vaguely named non-profit groups. Sunlight's Political Ad Sleuth, which makes the FCC files more easily searchable, has also exposed the endless nature of political airwars: The first ads in the 2014 Kentucky Senate campaign, which features Senate Minority Leader Mitch McConnell, R-Ky., vying for a sixth term, aired in February.

Sunlight's work with Political Ad Sleuth also revealed shortcomings in the current filing system, most notably the failure of many groups buying advertisements to identify their officers or principals, as required by the Bipartisan Campaign Reform Act. Monday's filing to the FCC made recommendations for how to address that and make it easier for the FCC to police the filings.

Members of the Public Interest Public Airwaves Coalition, which prepared the brief to the FCC that Sunlight joined, include: the Benton Foundation, the Campaign Legal Center, Common Cause, Free Press, the New America Foundation and the United Church of Christ Office of Communication Inc.