For years, net neutrality advocates warned that without government intervention, Internet-service providers would rig traffic flow for their own services or sell traffic priority to the highest bidder.

But they've had few examples of such abuse to point to, forced to settle on trying to whip up opposition with drab cases including a Canadian firm blocking access to a labor-union website during a strike and a North Carolina internet provider blocking a popular voice-over IP phone service.

That could be about to change. AT&T — itself a service provider — is unveiling a streaming movie and TV service next year in a challenge to Netflix, Hulu, and Amazon Instant Video.

With streaming video comprising about three-fourths of Internet traffic, net neutrality campaigners say AT&T has a clear financial motive to give its own service an edge, either by throttling competitors, accelerating the speed of its own product, or undercutting rivals on pricing, such as by discounting video streaming from data usage.

The behemoth telecommunications company provides Internet access in nearly two dozen states, and after acquiring Time Warner earlier this year has access to a vast array of popular video content. The video-player platform envisions hosting HBO and TBS shows and Warner Bros. films. The company also owns CNN.

AT&T says it’s not going to cut customer access just to boost its streaming service over competitors, but skeptics doubt it.

“This is a prime example of why strong net neutrality protections are so important," said California Democratic State Sen. Scott Wiener, author of a new state law seeking to resurrect Obama administration net neutrality regulations.

"AT&T is directly competing with the services that rely on AT&T’s network," said Wiener, whose law is being challenged by the Justice Department and Internet companies as part of a tangle of net neutrality litigation snaking through the courts.

“AT&T has every financial incentive in the world to make it easier to access its own product," Wiener said. "We shouldn’t have to rely on the ISPs' good faith."

An AT&T spokesman pushed back, arguing that limiting consumer choice would be bad for business — a line of argument advanced by Republicans on the Federal Communications Commission, who this year repealed Obama-era net neutrality rules.

“We participate in a fiercely competitive marketplace and must deliver to our customers the content and services they want, not because of any regulation that requires us to, but because that is what our customers demand and that is what makes good business,” the AT&T spokesman, Michael Balmoris, said.

“We support an open internet and have done so for more than a decade,” he added. “We believe Internet practices should be transparent. We will not block lawful Internet traffic, censor online content or throttle or degrade traffic based on content. We support customers’ rights to access content when they want and how they want.”

Balmoris highlighted AT&T’s support for expansive net neutrality legislation to establish an "Internet Bill of Rights.”

But the company’s words ring empty to Josh Tabish, tech exchange fellow at the pro-net neutrality group Fight for the Future.

Tabish notes that AT&T recently cut off broadcasting of HBO to 2.5 million subscribers on the Dish satellite TV network over a contract dispute. AT&T owns Dish rival DirecTV.

In another recent development, AT&T moved to cut off service for a small number of customers accused of copyright infringement.

“The power to block, throttle, and charge new fees for specific apps and services is their end game. But right now they know the entire Internet is watching their every move,” Tabish said.

"How vertically-integrated ISPs handle streaming services they own is going to be a serious litmus test for life without strong net neutrality protections. With no federal rules in place, they have every incentive to make competing apps, content and services more expensive — or block them outright,” he said.

