As Jay Leno and David Letterman continue to tussle over post-prime-time preeminence, a new breed of talkers led by Bill Maher are moving in

Prime-time programming may go into a prolonged snooze during the long hot summer, but nighttime talk shows just get livelier. Movie stars beef up the guest lists, couch hopping from Jay to Dave to Conan to promote their blockbusters; sunburned party animals home from college stay up late and ratings don’t fall off the way they do earlier in the evening. The result is that while you may go on vacation, NBC’s The Tonight Show With Jay Leno, CBS’ Late Show With David Letterman, NBC’s Late Night With Conan O’Brien, ABC’s Politically Incorrect With Bill Maher, and Comedy Central’s The Daily Show are busy working up a sweat.

By now, it’s old news that The Tonight Show has been whomping Letterman in the ratings pretty consistently for about two years. But a few things are new about this situation. One is that the Jay versus Dave brouhaha is no longer the primary story — rather, it’s that the late-night race is wide open, with ABC’s Politically Incorrect providing real competition. Another is that Dave, once the definition of cutting edge, is no longer perceived as the risk taker: Both Conan and The Daily Show are increasingly viewed as the most inventive.

The TV press and some of the late-night comedy writers interviewed for this piece agree that Conan O’Brien has improved in every way, from camera savvy to market share — and viewers seem to agree. (Conan is up half a million viewers since 1993, now averaging 2.5 million a night, while figures for Tom Snyder’s Late Late Show have remained steady at 1.7 mil.) As for Jay and Dave, it is commonly thought that there’s a crassness to their shows — flagrant on The Tonight Show, more tentative on Letterman — inspired by their head butting. O.J. Simpson, Frank Gifford, Marv Albert, and Paula Jones jokes abound; murder, adultery, and sexual peccadilloes have replaced Johnny Carson’s milder ”It was so hot today…” style of monologue material. One late- night writer who declined to be identified observes, ”It’s so weird that tasteless stuff is mainstream, but people do really get into it.”

Leno inspires a certain grudging admiration. Another writer who has toiled on a number of late-night shows believes that Leno’s show has ”definitely improved” and attributes much of that to The Tonight Show‘s set, which was built in September ’94 to allow for more audience interaction. ”The show is much better designed, whether it’s funnier or not.”

In comparing the two shows, this writer notes: ”First, Dave created this incredible energy at CBS, and the band [was] playing music in the monologue and punctuating every joke. Then Jay started doing the same things…always pump[ing] the audience, pimping them for applause. All this competitive energy creates pressure to get away from invention, because invention is risky.”

Invention, it would seem, is now the province of The Daily Show, where host Craig Kilborn is doing the most acerbic humor after dark, and particularly PI, whose famous-folks-arguing format is enabling it, ABC says, to beat the second half hour of Letterman in 17 markets. (CBS retorts that in the hallowed 18-34 demographic, however, Dave scores nearly a 2-to-1 ratings victory over PI.) Since Letterman has lost over 1.5 million viewers since 1995 and Leno’s gain has been about half a million, you have to figure that some folks are getting their presleep laughs from options like PI and The Daily Show.