The nation's broadcasters sued the Federal Communications Commission Monday over a rule that requires them to post political ads bought at some stations online.

At risk: a ruling designed to make information about political ad buys available via the Internet in time for the fall election. Sunlight has calls into both the FCC and the National Association of Broadcasters, the group suing the agency, for comment. So far, no response.

But Corie Wright, senior policy counsel for Free Press, and a longtime advocate of the ad buy rule, called the lawsuit "nothing more than an attempt by the NAB to stall an important and overdue transparency initiative" in a statement issued Tuesday.

The FCC had expected the rule requiring online posting of ad buys to come into effect before November and has been awaiting approval from the Office of Management and Budget. Once the rule is approved by OMB, stations would have to comply within 30 days.

The rule requiring stations to post files that the public now must visit stations to review on paper is "arbitrary and capricious," the NAB argued in its filing to the DC Circuit Court of Appeals, because it targets broadcast television stations and not competitors -- which, in prior statements, the NAB has identified as cable networks. The NAB argues that it the new FCC rule would "adversely impact NAB and the broadcasters whose interested it represents."

The NAB's main objections to the FCC rule: It could hurt stations' negotiating stance with advertisers who would be able to see ad rates, and it would require additional manpower to post the logs.

The rule that the FCC approved in April established a pilot program of sorts, ordering local affiliates of the four top broadcast networks -- ABC, CBS, Fox and NBC -- in the 50 most populous media markets to place their political ad files on a web site administered by the FCC. The rule would still exempt stations in much of the country including large swathes of battleground states such as Missouri, Virginia and Wisconsin. If would not, for instance,have captured over half of the ads run by three of the highest-spending political committees during a sample period in April, according to a Sunlight analysis.

Even so, it would represent a critical first step in a better-than decade-long effort to improve the transparency of political ad buys -- one that is particularly crucial in a year when new campaign finance rules will enable many non-profits to buy advertising without registering with the Federal Election Commission. That means paperwork filed with local television stations will provide the only clue as to who is behind the shadowy committees.

The Sunlight Foundation, in partnership with Free Press, the New America Foundation and other journalism and open-government organizations, is helping to organize an effort to put files from local televisions online and to create a searchable, sortable database so voters can learn more about the organizations that are trying to influence them. The effort launches this month with a pilot program in Wisconsin. To learn more about how you can help, check out our new Political Ad Sleuth page.