Americans get a new tool this week for approximating the size of what you might call the nation’s “Gross Political Product.”

Beginning Tuesday, every broadcast television station in the country will have to post online copies of contracts and other information about the political advertisements they are airing. For the first time, voters will have easy access to documents detailing who is buying campaign commercials and how much money they are spending.

The ability to take a close and more accurate read of spending on (mostly negative) political TV ads — already north of $40 million in April when the Wesleyan Media project released a study on this year’s Senate contests — represents a rare victory for transparency in a political system increasingly inundated with dark money. It’s also a huge expansion of a pilot project launched two years ago, when the Federal Communications Commission (FCC) ordered affiliates of the top broadcast networks (ABC, CBS, FOX and NBC) in the nation’s 50 largest TV markets to begin online posting of their political ad files.

That order covered about 230 TV stations; the one that takes effect this week will expand that number to more than 2,000. The Sunlight Foundation has made it possible to search and sort those files by advertiser, by date and by state with its Political Ad Sleuth tool.

The expanded online access comes just in time. At least six of this year’s most hotly contested Senate races — which will likely determine control of the closely-divided chamber — are taking place in states that don’t have a top-50 television market: Alaska, Arkansas, Iowa, Louisiana, Montana and West Virginia. In addition, the public will now be getting online access to the political ad files of important stations that aren’t affiliated with the big four broadcast networks, including Spanish-language stations.

Having the political ad files online is important. In some cases they provide the only public information available on groups that are thinly disguised as nonprofit “social welfare” organizations but are, in fact, major campaign players. Their tax status, plus the way they word and time their political advertising, exempts them from having to disclose their spending or their backers to the FEC, the agency created after the Watergate scandal to make campaign spending accountable.

An example is Americans for Prosperity (AFP), which has not reported a single TV ad buy during the 2014 cycle to the FEC, even though Sunlight’s ad tracking tools show that the group has been extremely active in swing states, especially North Carolina and Michigan. Are the ads AFP is running campaign ads? Not according to the letter of the law, but you can check out the videos captured by Sunlight’s Political Ad Hawk and judge for yourselves.

We don’t mean to pick on Republicans, either; though none of them are quite so assiduous as AFP about remaining undocumented, plenty of Democratic-allied groups, such as Patriot Majority and the League of Conservation Voters, engage in the same kind of under-the-FEC-radar advertising.

It’s legal to do because: Courts have said outside groups can spend unlimited amounts of money helping — or torpedoing — candidates for office as long as they don’t coordinate with them; and unless they expressly call for a vote for or against a candidate or they mention or depict a candidate and air within 30 days of a primary election or 60 days of the general, those ads don’t have to be reported to the FEC.

Also worth noting: Many of these ads are aired by groups that, because they are organized as nonprofit corporations rather than as traditional political action committees, never have to disclose the sources of their money.

So it’s very possible the ad you just saw cataloging the wonderfulness (or awfulness) of your member of Congress was brought to you by a mystery-meat group whose identity and agenda has been carefully hidden from view.

That’s why the Sunlight Foundation and other organizations, including Free Press and ProPublica, began an effort to systematically track TV ad buys during the 2012 campaign.

Until 2012, TV ad files were, in the words of the FCC order mandating online posting, “public more in theory than in practice.” While TV stations have been required to make political ad contracts since 1938, seeing them meant making in-person visits to the stations (generally open for this task only on weekdays during regular business hours) and poring over paper records. That made it virtually impossible to obtain and aggregate the records that would show patterns of influence-buying nationally.

The online files have enabled reporters at Sunlight and other organizations, to:

• Learn how early political advertising begins in key contests;
• Unmask the donors behind political ad buys; and
• Expose how television stations have been allowing groups to buy ads without disclosing the information required under federal law.

The last discovery, discerned only because the online posting allowed a systematic analysis of ad buys nationwide, prompted Sunlight, along with the Campaign Legal Center and Georgetown University’s Institute for Public Representation, to file a complaint with the FCC. That in turn led FCC Chairman Tom Wheeler to put stations on notice to do a better job of policing their customers. Which, we have noticed, the stations have been doing.

All of which goes to show that watchdogging makes a difference — watchdogging that will be greatly enabled when all of the broadcast political ad files go online.

If information is power, American voters have just gotten a big new turbine. Let’s use it.