NBC’s “Nightly News” remains the most-watched evening newscast in the United States. For Madison Avenue, however, that information may provide cold comfort.

For the second week in a row, ABC’s “World News” lured more of the viewers advertisers in news programming desire — people between 25 and 54 — than NBC’s “Nightly News.” The last time ABC trumped NBC in that category in the evening-news slot for two consecutive weeks came in October of last year, according to Nielsen.

To be sure, ABC’s margin of victory is slight: Its “World News” featuring anchor David Muir attracted an average of 1,941,000 viewers between 25 and 54 for the five days between March 16 and March 20, according to Nielsen. NBC’s “Nightly News” won an average of 1,935,000 during the period — just 9,000 fewer people on average.

Yet NBC’s newscast is under scrutiny in the wake of the suspension of anchor Brian Williams. The longtime face of the network’s news division, Williams is currently serving a six-month suspension after he disclosed that he falsified an account of a 2003 NBC News reporting excursion to Iraq. NBC News is conducting an investigation to determine whether Williams has told other canards during his tenure on the program.

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And while it is well known that NBC has a “Brian Williams problem” of sorts, advertisers are watching the evening-news ratings to determine if the network has a problem owing to the anchor’s absence. Under Lester Holt, who has anchored “NBC Nightly News” since Williams’ last broadcast on Friday, February 6, “Nightly News” has continued to lure the most viewers of any evening newscast, but it has been challenged by ABC in the demographic most important to sponsors of the programs.

ABC’s audience between 25 and 54 fell 4.5% from the prior week and 4% from the year-earlier period, according to Nielsen. NBC’s audience in that demographic is down 1.27% from the prior week, but 14% down from the year-earlier time span, according to Nielsen data. CBS’ viewership in the demographic averaged 1,538,000, Nielsen said, down 4.7% from the earlier period. CBS’ newscast was preempted twice during the period by basketball coverage.

The battle for that demographic was tooth-and-nail last week, Nielsen figures show. “Nightly News” actually won in the category three out of five nights in the five-day period — Monday, Tuesday and Friday, according to Nielsen — but lost on Wednesday and Thursday.

Among total viewers, NBC lured an average of 8,518,000, 4.75% more than the previous five-day period, while ABC attracted an average of 7,957,000, representing a dip of 2% from the prior week.

Season to date, “Nightly”has won an average of 9,326,000 total viewers, 6% more than ABC and 26% more than CBS. The NBC newscast has won an average of 2,228,000 viewers between 25 and 54, 2% more than ABC and 26% more than CBS, according to Nielsen.

The margins between NBC and ABC remain thin, and will no doubt be studied further by media buyers.