After two years on top, Brian Williams’s “Nightly News” broadcast has suddenly been eclipsed by ABC and Charles Gibson.

“It is predictable,” Mr. Williams said. “This is why I haven’t allowed any champagne toasts in the newsroom when the ratings have been flawless and spectacular and joyous. This is a back-and-forth dogfight.”

Tough competition keeps everyone sharp and benefits the viewers, he said.

ABC’s “World News” has been the most popular newscast for eight straight weeks, and 15 out of the last 19, according to Nielsen Media Research. Katie Couric’s “CBS Evening News” is a distant third.

All the attention paid to Ms. Couric’s tough start at CBS has overshadowed what’s been going on at NBC.

In Ms. Couric’s first 39 weeks at CBS, she’s lost 287,000 viewers from the average of a year ago, a drop of 4% from predecessor Bob Schieffer. At the same time, “Nightly News” lost 533,000 viewers, or 5%, Nielsen said.

In Mr. Williams’s first three months after taking over from Tom Brokaw in December 2004, “Nightly News” averaged 10.79 million viewers. In the past three months, it’s been 7.66 million.

“If I was at NBC, I’d really be quite nervous about the hundreds of thousands of people that have left my audience,” said Andrew Tyndall, a consultant who studies the content of network newscasts.

Mr. Tyndall credits ABC more than he faults NBC. In a sense, the loss of audience NBC should have anticipated with Mr. Brokaw’s departure was delayed for two years because of turmoil at ABC — the death of Peter Jennings, injury to Bob Woodruff, and departure of Elizabeth Vargas, he said. With Mr. Gibson, now there’s stability.

Mr. Williams, whose “champagne toasts” comment was a sly reference to bottles passed around ABC’s newsroom due to recent ratings triumphs, said he leaves it to others to worry about the numbers.

“I, honest to God, couldn’t tell you what the ratings are and couldn’t tell you that for days on end,” he said. “It really is immaterial in a way. There isn’t anything we can do on a given day to tweak.”

“We worry about what we can worry about, but our numbers are affected by a variety of things we have very little to do with, nothing to do with,” said Alexandra Wallace, “Nightly” executive producer for three months, who made a rookie mistake in predicting to Philadelphia Inquirer reporter Gail Shister in late winter that NBC would soon be back on top after ABC’s initial victories.

The simplest possible explanation, and toughest to deal with: More people want to spend that half hour with Mr. Gibson than with Mr. Williams.

Mr. Gibson, 64, is an older man anchoring a news format favored by older viewers. (Mr. Williams is 48.) He’s also more affable and self-effacing on the air, with less starch than the more formal Mr. Williams. Some see Mr. Williams as talking to, rather than with, his audience.

NBC has struggled to find a way to show, on the news, the warm, quick-witted Mr. Williams that appears in guest shots on “The Daily Show.”

“One man’s formalness and bearing is another’s being a guest in someone’s home and anchoring a broadcast that they’ve come to expect in a certain way,” Mr. Williams said.