The NFL's popularity seems to be declining. Image Source: ESPN. 

For years, football was the salvation of the U.S. television industry. As broadcasters like Walt Disney (NYSE:DIS)ComcastCBS (NASDAQ:VIAC), and 21st Century Fox (NASDAQ:FOX) (NASDAQ:FOXA) saw ratings slide due to cord-cutting and competing entertainment, football could still reliably deliver massive audiences.

The live nature of sports also means viewers will tune in at a set time, reluctant to DVR the game to speed through commercials, a boon for advertisers who are struggling to find ways to connect with consumers in the digital age. While much of network programming has seen ratings collapse, NBC's Sunday Night Football has been the most-watched show in the country for the past five years.

However, there are now signs that football's dominance over television may be waning.

Cracks in the armor 

With the NFL season almost halfway over, ratings are down 10% to 15% so far this year. It's unclear why viewership has fallen, but there are number of theories.

Some viewers have likely been sucked away by the contentious election season, which has taken interest away from games and shifted it to cable news, which has seen ratings soar. Past election cycles have cut down on football's audience, though never to this extent.

Other theories include the raft of negative press surrounding the NFL, including incidents of domestic violence and the growing perception that football is unsafe, exacerbated by accusations that the league has covered up evidence of permanent head trauma. A number of notable names, including President Obama, Lebron James, and even NFL Hall-of-Famer Mike Ditka have cast doubt on the future of the game and its safety by saying that they wouldn't let their own sons play the game.

Another possibility is that the league has simply saturated the market and overexposed the brand. NFL games are on all day Sunday, Monday night, and Thursday night. What was once a weekly ritual seems to have blanketed the small screen with games and corresponding coverage. And of course, entertainment options beyond the television set continue to get better.

What's at risk here?

All four major broadcasters count on the NFL to deliver millions of viewers this time of year. CBS and FOX carry Sunday afternoon games. NBC, owned by Comcast, hosts the popular Sunday Night Football, and ESPN, a division of Disney, has nabbed the long-standing Monday Night Football franchise.

Ad revenue is tied to ratings, and the networks have shelled out big bucks to carry NFL games, meaning the networks may not see a sufficient return on their investment. According to some sources, the networks see football as a "loss leader" to promote other shows and lift the network's overall brand. 

CBS, NBC, and Fox are contracted through 2022 to pay a little more than $1 billion per year for the rights to carry their set packages of NFL games. ESPN, on the other hand, is paying $1.9 billion annually to air Monday Night games through 2021. The Disney subsidiary only gets 17 games in its package, fewer than its three rivals, and because those games are on cable, they bring in smaller numbers of viewers. According to Bloomberg, those differences mean ESPN pays about four times as much as the broadcast networks per viewer.

As a result, declining ratings are a particularly troubling problem for Disney and ESPN. The No. 1 cable network relies on carriage fees from subscribers in addition to ads for its revenue. But fading interest in football could mean an even sharper decline in subscribers for the sports media powerhouse. And unlike the broadcast networks, ESPN depends on football interest for other programming, such as its flagship news show Sportscenter and talk shows throughout the day, meaning ratings around the clock could be lower.

ESPN is still Disney's biggest cash cow -- media networks deliver nearly half of the company's operating income, and the ESPN family of channels, which includes ESPN 2, ESPN News, ESPN Classic, among others, is by far the biggest component of that media division.

It's too soon to say if the decline in NFL viewership will be sustained, but if continues, ESPN's profits will see even more downward pressure, and Disney may have to redirect its strategy for the sports entertainment juggernaut.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.