After an early period of see-sawing ratings, the competition between David Letterman and Conan O'Brien is falling into a nightly pattern that is relatively unaffected by repeats of either of the late-night shows' hosts, the New York Times observed today (Friday). "Older viewers don't watch Conan O'Brien, even when David Letterman is in repeats; and younger viewers don t watch Mr. Letterman, even when Mr. O'Brien is in repeats," the newspaper concluded. Meanwhile, Advertising Age , reported today that ABC has launched "a short tactical campaign" to alert viewers that its Nightline is beating both Letterman and O'Brien among overall viewers. And while the numbers are close for all three programs, a wider gap could open next month with the debut of the nightly Jay Leno Show on NBC at 10 00 p.m. Noting that last week's ratings were up 35 percent over the comparable week a year ago, Nightline executive producer James Goldston told AdAge , "Our feeling is that we've got some momentum and now's the time to just kind of Take That message out and remind people about the show."