Is he too cranky? Too ironic? Too . . . Dave?

Increasingly, there's reason to wonder. David Letterman is slipping, sliding, flailing. It's no longer Jay vs. Dave in the late-night TV battle. That's over. Leno's "Tonight Show" has beaten Letterman's "Late Show" in the Nielsen ratings in 200 of the past 205 weeks, or for almost four consecutive years. Nowadays, Letterman finishes well behind Ted Koppel's "Nightline" at 11:35. He's even wheezing to stay ahead of Conan O'Brien's talk show, which starts at the red-eye hour of 12:35 a.m.

Since Letterman's vaunted switch from NBC to CBS in 1993, "Late Show" has shed nearly double-digit hunks of its audience every year. This past season, the number of people watching "Late Show" shrank by another 10 percent. Which means Letterman now attracts an audience that is less than half the size of the one he started with on CBS.

The losses came despite CBS's rise to No. 1 in prime time and the network's resumption of NFL telecasts. In theory at least, football and "Touched by an Angel" should have reversed Dave's descent, since the big audiences watching those programs were being bombarded with "Late Show" promos. Instead, with an average of 3.5 million viewers a night, "Late Show" is now drawing only 200,000 more viewers than Pat Sajak's famously disastrous late-night talk show on CBS in 1989-90 (although Sajak admittedly didn't have as much cable competition in his day).

Locally, the news isn't much better. Letterman, on WUSA, is far behind Leno on WRC. But he also loses by a wide margin to "Nightline" on WJLA, and even to musty reruns of "M*A*S*H" on Fox affiliate WTTG. "Late Show" would be the fifth choice in the Washington area at 11:35 had WDCA not decided to move "Jerry Springer" from 11 to 10 p.m.

Officially, CBS executives express no alarm. Financially, they say, the show remains strong with advertisers, who pay premium prices to reach Letterman's generally affluent viewers. Further, they point to last year's Emmy Award (and 37 previous Emmy nominations) as evidence that Letterman's show remains creatively strong.

"For us, it starts and ends with the quality of the show," says Mitch Semel, CBS's senior programming executive in New York. "Of course, we're in the communications business, and we're cognizant of the ratings. But we know we have the best show in late night."

Leno, Semel says, "is a good hamburger. Dave is a great steak dinner."

Nevertheless, Letterman's sagging fortunes are drawing serious attention from CBS. The network has begun "a fairly exhaustive" research project to analyze the late-night audience and how to bring more of it to CBS, says David Poltrack, CBS's top research executive.

"We have a solid leadership position in daytime, our prime time is strong and getting stronger, so now we can focus on" turning around late night, he says. "Obviously, this is a trend that has to be reversed or flattened out."

Letterman's reps (Letterman himself was unavailable for comment) point out there are some factors he can't control. Namely, there's a mismatch between CBS's prime-time audience (it's older, with an average age of 52--the same as Letterman himself), and the average "Late Show" viewer (who is 43). What's more, Letterman's lead-in--the late local news--is relatively weak on many CBS affiliates; "Late Show" beats Leno in the few cities (such as New Orleans, Kansas City, Houston) where the CBS station has the leading newscast.

"The frustration is, we feel misplaced," says Rob Burnett, "Late Show" executive producer. "We have no frustration with the people running the network. . . . But it's clear that the people watching the network [in prime time] are not the people flowing into the 'Late Show,' " or Craig Kilborn's talk show at 12:35 a.m. "The people [CBS] is promoting our show to will not watch it."

Indeed, Letterman's support has all but collapsed among younger viewers, whom advertisers covet most. In five years, he's lost about two-thirds of his young-adult (18 to 34) audience, according to Nielsen. "Late Show's" ratings among these viewers were so dismal last season that Conan O'Brien, airing an hour later on NBC, tied Letterman in this age segment. CBS's brass believes that the big hurt is coming from rival stations that air syndicated reruns of "Frasier," "Friends" and "Seinfeld" during Letterman's time slot; these shows have strong appeal for young viewers, especially men.

Of course, there's an alternative explanation, one that those close to Letterman flatly reject: Maybe Leno just puts on a better show.

"Early on, Jay didn't have his groove, he was uncomfortable, and Dave was killing him," says Marc Berman, a New York media consultant and former "Tonight Show" researcher. "Then NBC started tinkering with it. They changed the set. They changed the format around, the mood of the show. It was more natural for Jay, and the audience responded."

Letterman's audience began to decline, Berman says, around the time of his widely panned hosting of the Academy Awards telecast. ("Uma, meet Oprah. Oprah, Uma. Oprah . . . Keanu.") Now, Berman adds, CBS "can't really take the show and make it a different show. Letterman is who he is. To make him something that he is not would really make people tune out. The ball is in Leno's court, and I think it will be there for years."

Don Ohlmeyer, the former NBC executive responsible for the changes in Leno's version of the "Tonight Show," says the relative fortunes of Leno and Letterman are "a direct result of Jay Leno being able to run the marathon better than anyone. Letterman took the lead at the five-mile post. But where he hit the wall of pain, Jay was able to go through it. . . . The fact is, the public was watching Jay, it shifted to Dave, and then it gradually came back to Jay. It's not a rejection of Letterman so much as it's a reflection that Jay puts on a better, funnier show, one that seems more current and is more in line with what people are doing and thinking at 11:30 at night."

In a late-night talk career that stretches back some 17 years, Letterman surely ranks among television's most gifted performers, and one of its most enduring. It's fair to say that his late-night shows, first on NBC and continuing on CBS, have not only redefined the traditional talk genre, they've defined an entire style of humor--self-deprecating, sarcastic, ironic, skewed.

Beyond the standard celebrity interviews and monologue, Letterman's shows have introduced idiosyncratic pieces of "found" humor. He (or guests) tossed objects off tall buildings just to watch them go splat; he regularly sends cameras onto the streets surrounding his Manhattan studio just to bump into something funny; his show has also used street characters and stagehands as ironic performers. And Letterman's signature routines--the Top Ten List, Stupid Pet/Human Tricks, etc.--have become genuine pop culture hallmarks.

But the host now seems to be at a career crossroads. Although Letterman just re-upped for three years on CBS, "Late Show" producer Burnett isn't sure what lies beyond that. "That's a difficult question for anyone to answer," he says. ". . . He's done this for a long time. He enjoys it, to some extent. There are some things he finds less enjoyable. It's a personal decision and he hasn't made it."

He suggests that Letterman might take a more active role in his production company, Worldwide Pants, which produces "Late Show," "The Late Late Show" with Kilborn and "Everybody Loves Raymond" for CBS.

"Like anything you've done for 17 years, clearly there are some things that are less exciting now," Burnett says. "Dave is driven first and foremost by creative needs. As long as the show fulfills his creative needs, the show will continue. By and large, his needs are fulfilled right now."

But maybe not forever. Turning to the camera last night, Letterman offered a classic Dave-ism: "Paul [Shaffer] and I are working very, very hard to make it look like we're having a good time here."

These days, such a statement might raise a question. Is he only kidding?

CAPTION: CBS took a Lettermanesque approach to an NBC billboard in New York promoting Jay Leno's "Tonight Show."

CAPTION: David Letterman with guest Jerry Seinfeld, whose comedy series reruns are competing for ratings with the "Late Show."