Major broadcasters will be required to post documents detailing who is purchasing political ads and how much they cost, following a Federal Communications Commission vote Friday.

The panel approved the measure 2 to 1, with the Democratic commissioners in favor and the lone Republican opposed.

The new rule will require TV stations to submit records to the FCC Web site showing the purchasers, cost and frequency of political ads run by their stations, information they now must maintain in paper records available if requested in person.

Democrats have assailed the growth in political spending by interest groups in recent years, which has largely favored the GOP. The FCC vote is the latest in a series of moves by Democrats to increase disclosure of political donors and spending on campaign advertising.

The debate over the rule pitted transparency advocates against broadcasters, who resisted publishing pricing information. In the end, neither side was fully satisfied with the outcome.

Transparency advocates said the newly available documents will help the public know who is spending money to influence campaigns in an election that could draw $3 billion in broadcast political advertising. But the advocates were displeased with the commission’s go-slow approach, which limited the new rule to just the largest broadcasters.

“It’s a little discouraging that this is as far as they went today,” said Lisa Rosenberg, a lobbyist with the Sunlight Foundation, which advocates for transparency in government and politics. “By law, this information is supposed to be public. In this day and age, in the 21st century, if it’s not online it’s not public.”

Rosenberg also said the files will be of limited use because, while they will be in one location on the FCC Web site, they will not be in a database format that can be easily analyzed.

FCC Chairman Julius Genachowski said before the vote that the commission staff spent 61 hours and incurred nearly $1,700 in copying costs to get the public file from eight stations in Baltimore.

Broadcasters say if their rates are easily available online, other advertisers will demand to pay the lower prices charged to campaigns. By law, broadcasters must offer campaigns the cheapest rate available to any advertiser.

“By forcing broadcasters to be the only medium to disclose on the Internet our political advertising rates,” the National Association of Broadcasters said in a statement, “the FCC jeopardizes the competitive standing of stations that provide local news, entertainment, sports and life-saving weather information free of charge to tens of millions of Americans daily.”

The disclosures are not likely to affect political campaigns, which already have access to the same information through media buyers, who communicate with the stations.

“It’s not going to provide any more information to the good campaigns,” said Brad Todd, a veteran GOP media consultant. “The good campaigns already have competitive information.”

The rule will go into effect 30 days after it is published, which is likely to happen before the November elections. But the requirement will apply only to stations in the top 50 media markets and the top four networks. The commission will evaluate the program after two years and could decide to include smaller markets or to rescind the requirement and move back to paper records.

The move at the FCC is only one of several promoted by Democrats to require more disclosure of the flow of money to interest groups since the landmark 2010 Supreme Court decision in Citizens United v. Federal Election Commission. The party and its allies have pushed legislation, filed lawsuits and asked for investigations into interest groups who do not disclose their financial backers.