For the past few years, turning on ABC's "Good Morning America" was like happening upon a joyous party. Robin Roberts, George Stephanopoulos and their pals all seemed nice, and looked like they loved being together. It was almost always a good time.

The party isn't over. But it's getting late.

Television's top morning show is in its first prolonged slump since overtaking NBC's "Today" in popularity three years ago. "GMA" is losing viewers, most rapidly among a younger demographic that advertisers eagerly seek, where NBC has wrested back the lead from its rival.

The search is on for those missing viewers, many of whom ABC executives suspect have turned off morning television altogether, and for ways to lure them back.

The "Good Morning America" typical daily audience is 4.9 million, or 11 percent smaller than last season, the Nielsen company said. Among 25-to-54-year-olds, the decline is 19 percent. "Today" now leads among that group not because it is surging — the NBC show is down 4 percent from last year — but because it is losing audience more slowly. The demo number is the key to a vault, since advertisers check it first when making spending decisions.

"Today" is feeling bullish. Its team is clicking, with Matt Lauer appearing more comfortable than he has in years. The show's aggressiveness in seeking interviews has paid off, most recently with Charlie Sheen's revelation that he is HIV-positive.

But ABC's bigger concern may be smartphones on the bedside table. Morning television usage peaked at 40.7 million viewers three years ago and has dropped by two million since, Nielsen said. Some people who used to turn on TV for a quick check of the headlines and weather now likely do that online.

To that end, "GMA" executives are emphasizing live events and features to convey a sense that people will be missing out on something important if they don't turn on the TV. They turned to social media, for example, to heavily promote Roberts' interview with Kobe Bryant this week.

"Good Morning America" has suffered turnover the past couple of years, with Josh Elliott leaving for NBC Sports and Sam Champion for The Weather Channel. The short-term ratings impact was minimal, but long-term may be different.

"It's really who you want to spend time with for two hours in the morning," said Brad Adgate, an analyst for Horizon Media, "and they may have done something to change the mix of what people are comfortable with."

One thing beyond a television producer's control is the cultural zeitgeist, and a sense that a program is no longer in tune with the times.

To some, the "GMA" celebration of its 40th anniversary last month less than a week after the Paris attacks felt jarring. This is a serious, newsy stretch of time, with people on edge over terrorism and mass shootings on an endless loop. Next year's presidential election has attracted unprecedented attention a year in advance. Morning shows are a combination of news and entertainment, and nothing annoys ABC executives more than a suggestion they don't have the news chops to compete. Yet while "GMA" ratings are down this season, the more sober and traditional "CBS This Morning" is up 7 percent in viewers.

"The show should become George-centric again," said former ABC and CBS morning show producer Shelley Ross, suggesting Stephanopoulos' political expertise is an asset for an election year.

The risk in shifting to a heavier news emphasis is that the people who came to "GMA" because it looked like the cast was having fun and was quickest to laugh at the latest viral pet video may say, "this isn't what I signed up for."

ABC executives aren't talking publicly about what's going on with the show and, privately, they're not fully sure what has happened to the "Good Morning America" audience. They don't feel it is a rejection of the show.

"We never take for granted the position that we're in and we work every day to bring our viewers a smart, innovative show to start their day," Michael Corn, senior executive producer of the broadcast, said in a statement. "We are laser focused on this mission and making 'GMA' essential to our viewers."