The ascension of ABC’s “World News Tonight” began even before David Muir took the anchor desk in September. (Jabin Botsford/The Washington Post)

Little by little, ABC’s “World News Tonight” has climbed to the rocky summit of Mount Nielsen. After years of also-ran status, the evening newscast has caught up to NBC’s “Nightly News,” the ratings champ for more than five years. The two programs now jockey for the title of America’s most-popular TV news source.

How did that happen? NBC’s six-month suspension of its lead anchor, Brian Williams, in February probably didn’t enhance “Nightly News” public relations. But “World News Tonight” was gaining even before Williams tripped and tumbled over his bogus war stories.

The story of “World News Tonight’s” ascension is a bit more nuanced, starting even before ABC decided to replace anchor Diane Sawyer with rising star David Muir in September. At the same time, the venerable broadcast has slowly evolved into a newscast engineered for the social media age. In important ways, “World News” looks and sounds different from its competitors. It’s brighter, tighter and indeed quite a bit lighter than its evening rivals.

Under Sawyer, “World News” became noticeably softer, with a greater emphasis on celebrity and entertainment stories, weather coverage, crime fare, news-you-can-use and YouTube’s hottest videos. The trend has continued, and perhaps accelerated, with Muir, 41, at the anchor desk.

News from Washington — a staple of the broadcast since its Peter Jennings glory years — now fights for air. It usually loses: “World News” devoted half as many minutes to Washington stories as CBS did during the first four months of the year, and about 40 percent less than did NBC, according to Andrew Tyndall, who tracks the networks’ newscasts through his eponymous newsletter.

In perhaps a first for a national newscast, “World News” no longer has a full-time correspondent reporting on Congress. Such stories are handled on an ad hoc basis by reporter Jonathan Karl, whose primary beats are the White House and political campaigns.

Although Muir has anchored from Cuba and the Middle East, there’s far less world news on “World News Tonight,” too. During May, the broadcast led with domestic news almost every night of the week, despite a flood of developments in Syria, Iraq, Europe and elsewhere.

The news on “World News” can be serious and important, but the serious stuff is often fleeting. On a typical Tuesday broadcast in May, the lead story was a “breaking” report about a missing U.S. military helicopter in Nepal (the “breaking” element was unclear, given that the helicopter had been reported missing about 30 hours earlier). The second story was a report about a commercial jet’s “scary landing” (as a bold graphic labeled it) in Hawaii. The event produced no injuries, but the story did have the kind of visual element — video of a damaged plane — that made it compelling for “World News.”

The last quarter of the broadcast included amateur video of a man being extricated from a car involved in a crash, a home invasion caught on a security camera and footage of two men flying jet packs in Dubai.

It’s not just what “World News” covers that sets the broadcast apart, but how quickly it covers it. By design, Muir’s newscast has a faster and more urgent pace than those of his predecessor and rivals. According to Tyndall’s statistics, the average correspondent’s news report on “World News” was just 100 seconds last fall, compared with 138 seconds on NBC and 121 on CBS.

The breathless quality comes across in the newswriting and reading. Muir and “WNT’s” reporters frequently eschew verbs, replacing them with gerunds in a kind of headline-style speed talk. To wit: “And tonight, flash flood watches across the plains . . . Black Hawk helicopters airlifting families to safety,” the anchor intoned at the start of another weather story.

ABC executives acknowledge that their newscast is different but say that’s a positive development. The faster pace and sometimes offbeat story mix are designed for an audience that is already saturated with news, said Almin Karamehmedovic, executive producer of “World News Tonight.” “By 6:30, when we come on the air, most of our audience may have heard about” the major stories of the day, he said. “We’d like to give them a full picture . . . and to put it all in perspective.”

He adds, “I think it’s important to have a style that is unique. Not different to be different, but simple and clean and without confusion. . . . We’re fully aware of how people consume news these days, and we’re addressing it. There are a lot of Web sites and channels. It’s still important to have a place that can tell you the day’s stories in a way that’s accurate and urgent and in context.”

Karamehmedovic, a longtime international news producer for ABC, took over the broadcast in August 2014 during Sawyer’s waning days as anchor. Sawyer had made progress in denting “Nightly News” dominance during her five years as “World News” anchor. But it wasn’t until Muir’s ascension to the anchor chair in September that the ratings needle really began to quiver. “World News” has since added about 500,000 viewers to its total. It now regularly beats all comers among younger viewers (those ages 25 to 54), a key group for advertisers, though “Nightly News” maintains a slight advantage among viewers of all ages.

The Sawyer-to-Muir transition may have been the most important element in ABC’s ascension, said an NBC news executive, who spoke on the condition of anonymity because he’s not authorized to speak for the network. “As long as Diane was on the show, they were not going to win,” he said. “When David Muir filled in [for her], bang, they’d win the day. I can tell you that ‘Nightly’ was dreading the moment Diane retired” and Muir took over.

In the meantime, ABC News has lost some of its most experienced journalists to CNN. The list of departures over the past three years includes Bill Weir, Chris Cuomo, John Berman and two Washington mainstays, Jake Tapper and Jeff Zeleny. Virginia Moseley, an 18-year ABC veteran, left in late 2012 to become CNN’s deputy bureau chief in Washington. Christiane Amanpour went back to the cable network in late 2011 after a short stint hosting “This Week,” the Sunday-morning program with George Stephanopoulos.

ABC News wouldn’t comment on the departures but said that the network has hired a number of people from CNN during the same period.

Several former ABC News journalists described various states of disappointment about the changes at “World News.” Speaking on background, they lamented the rapid-fire presentation, the emphasis on visually arresting but largely vapid videos, and the diminution of news from Washington and abroad.

Among the more positive alums was former ABC News president David Westin, who installed Sawyer, the former “Good Morning America” co-host, as “World News” anchor in 2009.

“They’re putting on a program more people want to watch,” said Westin, who now runs a media investment advisory firm. “The audience gets a vote every night and they clearly like David Muir. And there’s a lot to like about him.”

Westin noted that the media landscape has changed dramatically even in the four years since he left ABC News, and that “World News” is attempting to craft a newscast that keeps pace. “The rules that applied in the 1990s don’t apply now,” he said. “Part of the problem here is that none of us have come to terms with what news is in this digital age. In the digital age, everyone constructs his or her own news from a wide range of choices. The landscape is more complicated than ever.”

Even so, the national evening news format has shown surprising endurance; the overall audience for the Big Three network newscasts has actually grown the past two years and during three of the past five years. It averages around 24 million people per night.

That means the networks’ long competitive fight for news viewers is far from over, said Deborah Potter, a former network correspondent who runs NewsLab, a research and training organization. And what the ratings suggest, she said, is that “World News” has figured out that a lighter approach may be a key to appealing to younger people who “are most likely to be tracking news through the day by other means on other screens.”