TV’s longest-running A.M. news battle is entering a new and potentially critical new phase.

“Today” and ABC’s “Good Morning America” are locked in a neck-and-neck race for dominance among the eggs-and-O.J. crowd, and their split has been stratified for months. For more than three years, “GMA” has typically commanded more viewers overall, while “Today” consistently dominated the viewer category that counts the most: people between 25 and 54. That’s the demographic most coveted by advertisers who sponsor morning programming; the show with the better numbers can demand better rates from advertisers.

Yet for the past two weeks, the dynamic has gone topsy-turvy: “GMA” has won more viewers in that all-important category, while still maintaining its overall audience lead.

“GMA” is making gains in the dead of summer. Its late-August run comes as many regular morning viewers go on vacation or shed their usual viewing habits. But it also takes place as “Today” has had to contend with a stretch during which viewers are not seeing its regular anchor team of Savannah Guthrie and Hoda Kotb (though “GMA” anchors have taken vacations in recent summer weeks). Kotb has been on maternity leave and is scheduled to return to the program after Labor Day.

To be certain, the contest between the two shows remains a squeaker. For the five days ended August 23, “GMA” won an average of 1,119,000 viewers between 25 and 54 – just 7,000 more than its NBC rival. Among overall viewers, “GMA” captured an average of 3.721 million, compared to 3.592 million for “Today” – a difference of just 129,000.

But if “GMA” continues its current trajectory, it could change the economic structure of one of TV’s most reliable – and lucrative – dayparts. Advertisers spent $1.13 billion on the three broadcast-network morning news programs, according to data from Pew Research – up 3% from $1.09 billion in 2017. Despite the bigger overall audience for “GMA,” however, “Today” tends to bring in more ad dollars annually. In 2017, for example, “GMA” won approximately $359.1 million in ad dollars for ABC  according to Kantar, a tracker of ad spending. That figure is significantly lower than what NBC gets from the first two hours of rival “Today,” which have in the past nabbed more than $500 million in a year.

In the Manhattan halls of NBC’s 30 Rock headquarters, executives have noticed that “Today” demo ratings have ebbed just after AT&T has blacked out local stations owned by Nexstar Media Group on its DirecTV satellite service as part of a larger battle over carriage rights. The belief at NBC, according to one person familiar with the matter, is that the skirmish has affected NBC stations more than it is those affiliated with ABC, and could be keeping as many as 10,000 viewers in the advertiser demo from watching “Today.”

NBC will hope Kotb’s return will bring “Today” viewers back to the fold, while the launch of the next season of “Sunday Night Football” will funnel bigger crowds to the show’s early-week broadcasts. At ABC, “GMA” keeps moving along. Even a recent controversy involving anchor Lara Spencer has done little to keep viewers from tuning in.

Producers at both programs may have thought they’d gain a reprieve from their ever-present ratings fight as the nation headed into the Labor Day weekend. Instead, they find that their struggle has begun in earnest – even before the new fall season arrives.