News media that rely on ads have always had a problem covering their own advertisers. It’s rare to find a reporter who doesn’t have a story, sometimes well-founded, of an employer whose newsroom pulled its punches or looked the other way to avoid rattling the worthies who paid the bills.

Obviously that’s bad, a corrupt concession to an institutional conflict of interest. Still, at least the harm was confined: The advertiser usually had narrow concerns — say, a car dealer that wanted to squelch some sour publicity. Killing the story was hardly a proud moment for the Fourth Estate, but the ad dependency didn’t shackle the media to a generalized, paralyzing incapacity to cover realities that lay at the core of civic life.

But suppose the ads don’t come from mere local retailers. Suppose they’re from people who bankroll elections. Welcome to 2012, where the sources of the money that’s critical to the business success of influential news media are, at the same time, the people who are orchestrating the major campaigns — people who, if news media were covering the news, would be confronted, exposed, and made to explain who they are and what they’re up to.

Instead, some of the same media that should referee political discourse and oversee the process by which a sovereign electorate selects its leaders are in thrall to the backroom players whose mission it is to manipulate and game that discourse.

The focus here is on local TV broadcasting, the most pivotal and most sought-after medium for targeting voters in battleground states. In an otherwise lackluster year for overall advertising, outlays on local TV are projected to grow 15 percent this year over 2011, thanks to TV’s disproportionate share of the torrential $3.3 billion in political advertising expected by Nov. 6.

A disquieting study by Timothy Karr of Free Press, a media watchdog, examined campaign ads on local TV affiliates of the NBC, ABC, Fox and CBS networks in Tampa, Milwaukee, Las Vegas, Cleveland and Charlotte. Those are second-tier markets, but they are the places that will swing the coming presidential election, and spending there has soared.

(The bonanza has also affected such mid-sized communities as Colorado Springs, where slippage in traditional Republican dominance could harm the party’s chances of holding Colorado. National Public Radio reports that spending there is three times what it was in 2008, when a 30-second local TV spot that normally cost $300 went for $7,000, and the city is now among the top 10 ad markets in the country.)

But Karr wanted to know whether the TV stations that are pocketing the money are also reporting on the entities that bankroll those ads, and whether they are checking the accuracy of the messages that have become the public’s principal source of political information. Are the media still practicing journalism, or are they nothing more than conduits for paid propaganda?

The Free Press findings were dispiriting. Network TV affiliates did no fact-checking on any of the political ads placed by the entities spending the most money in Las Vegas, Charlotte, Cleveland, Milwaukee and Tampa. In Cleveland, TV stations ran some 500 anti-Obama attack ads without any reporting about their chief funder, Americans for Prosperity, funded by the hard-right Koch brothers. In Charlotte, where the three top-spending funders had spent more than $4 million in the previous nine months, none of the stations offered any insight into the identity or objectives of the paymasters.

In a later look at Denver, site of the first presidential debate, Karr found that stations were getting a total of $6.5 million to air 4,954 ads from the five top-spending political action committees, while devoting less than 11 minutes to examining their truthfulness: a ratio, he concluded, of 162 minutes of campaign ads to every minute of related news.

In other words, the funders of political advertising appear to have purchased not just air time, but immunity from media scrutiny.

The movement toward aggressive fact-checking, which evaluates the utterances of politicians for accuracy and consistency, doesn’t seem to have traction in local newsrooms, at least not when the air time has been bought and paid for.

In fact it is there that the fact-checking obligation is the greatest, because it’s there that the overwhelming preponderance of political speech takes place.

It’s too much to expect media to turn down top-dollar ads that fail an elemental smell test, but they can at least make it clear that what the politicos are paying for is a right to speak for themselves, not a right to silence others.