Continuing a trend that began at the start of the summer, ABC’s “Nightline” has finished ahead of CBS’s “Late Night with David Letterman” in total viewers and adults 25-54 for four out of the past six weeks now, after lagging well behind for the better part of the last few years.

That raises the obvious question, is “Nightline” really doing that well, or is “Letterman” really slipping? The answer seems to be a bit of both.

For the week ended Sept. 10, the most recent available, “Nightline” averaged 3.56 million total viewers, 230,000 more than “Letterman.” In 25-54s, “Nightline” averaged 1.74 million viewers, 30,000 better than “Letterman.”

That comes three weeks after the ABC nighttime news program beat CBS for three straight weeks in both demographics for the first time since May 2003.

Certainly one would expect “Nightline” to grow entering a very contentious fall election season, when candidates and political talking heads will be popping up on the telecast almost nightly to talk up their race. And with the increasingly dire news in Iraq, along with the recent Sept. 11 anniversary, news shows may be temporarily more appealing than late-night comics yukking it up.

And “Nightline” is growing. Its total audience is up about 9 percent since former anchor Ted Koppel left the air last November.

But another factor is also “Letterman’s” continuing slide just as CBS recently re-upped his contract through 2010, one year after NBC rival Jay Leno leaves the air.

“Letterman’s” averaging 4.1 million viewers this season, down 5 percent from last year’s 4.3 million. In the younger demos where he used to be strong his fade is also apparent, down 7 percent, from a 1.4 to a 1.5 rating.

Barring a surprising turnaround, Letterman’s numbers could keep dropping till he retires, and the increasingly relevant question is who will CBS get to replace him then. Unlike NBC, they have not been grooming a replacement; “Late Late Show” host Craig Ferguson is too goofy and unless Katie Couric starts really tanking on the news, the network has no other well-known personality to plug in.

By giving Letterman a contract extension, CBS essentially bought the next four years to think about that, but in the meantime “Nightline” may make even more gains as CBS waits.

Meanwhile, for the week ended Sept. 10, NBC’s late-night offerings continue their reign, with “Tonight Show with Jay Leno” delivering its biggest ratings since the week ended July 28 and Conan O’Brien drawing his largest audience since May. Leno’s season-to-date lead over CBS’s “Letterman” grew to 38 percent both for 18-49 and total viewers. “Tonight Show” averaged 1.8/8 among 18-49s, beating “Late Show’s” 1.1/5 and “Nightline’s” 1.1/5.

NBC’s “Meet the Press with Tim Russert” was the Sunday winner with guest Dick Cheney. The program scored a 2.6 rating and 8 share in households, bettering CBS’s 2.1/6, ABC’s 1.8/5 and Fox’s 1.0/3.

In syndication, Oprah Winfrey’s road trip propelled her show to second place with a 5.3 rating among households, beating perennial No. 2 “Jeopardy’s” 5 rating and second only to “Wheel of Fortune” at 6.8. “Entertainment Tonight” tied “Everybody Loves Raymond” for fourth, with a 4.9 rating for each.

In daytime dramas, CBS was No. 1 in total viewers with an audience of 3.98 million, but ABC lead in the key women 18-49 demographic with a rating of 1.8, keyed by Rosie O’Donnell’s debut on “The View.” CBS came in second among women 18-49 with 1.7 and NBC finished third with 1.5/9.

And for the week ended Sept. 17, nightly news audiences were down for all broadcast networks except CBS, where Couric retained her lead despite falling from her debut week.

The Couric-helmed “CBS Evening News” was the No. 1 network news program for its second week in a row in both total viewers and adults 25-54, marking the first time CBS has been alone in first for two consecutive weeks in that demo since 1987. In 25-54s, “CBS Evening News” posted gains of 17 percent year-to-year to average 2.1, while NBC decreased 17 percent to a 2.0 and ABC lost 13 percent to fall to 2.0.