After ending a flirtation with ABC last year, Letterman wanted CBS to help his show by promoting it more heavily on outlets popular with young viewers and boosting the network's performance at 10 o'clock each weeknight.
The theory was that if more people watch CBS at that hour, they'll watch the local news and stay tuned for Letterman's "Late Show" instead of changing the channel for Leno's "Tonight" show on NBC.
CBS responded. And largely because of the hits "CSI: Miami" and "Without a Trace," CBS' viewership at 10 increased by 14 percent during the just-concluded TV season. NBC's audience dipped by 15 percent.
Yet Letterman's viewership dropped this year by 2 percent, and the gap between him and Leno remained virtually unchanged.
And despite reaching out to younger viewers, Letterman's audience among the 18-to-49-year-old demographic shrunk even more. Leno's lead widened from 29 percent to 39 percent, according to Nielsen Media Research.
The numbers are endlessly frustrating for Letterman staffers, who are justly proud of their five Emmy Awards for best variety series.
They say they're pleased with progress made this year; NBC just tries not to gloat too openly.
"It's no surprise," NBC Entertainment President Jeff Zucker said. "America decided this long ago and Jay's lead is only getting bigger."
Rob Burnett, executive producer of the "Late Show," is convinced more Americans would watch Letterman if there were an even playing field.
In 59 television markets where the CBS local news beat or equaled NBC's in the ratings during the May "sweeps," Letterman outrated Leno in 39 of them, he said. In the 138 markets where NBC's news was on top, Leno led in 132.
The failure of "Late Show" to capitalize on CBS' improved 10 p.m. performance doesn't mean the theory was wrong, he said.
It may just take awhile to pan out, since news-watching habits are slow to change, he said. The ratings for CBS' local news broadcasts improved by 5 percent this year.
Burnett likened the "Tonight" show advantage to the ratings success NBC had for years with lackluster Thursday comedies lucky enough to follow "Friends" on the schedule.
"The `Tonight' show is the `Veronica's Closet' of late night," he said.
Zucker said Burnett is "grasping for straws. That's the argument that they would make because they have no argument."
When they first went head to head, Letterman beat Leno in the ratings. But then numerous changes were made on "Tonight": replacing bandleader Branford Marsalis with Kevin Eubanks, adding goofy acts like the Dancing Itos, redesigning the set and extending the monologue.
Then in July 1995, Leno snagged Hugh Grant's first major appearance after Grant's arrest with a prostitute. Ratings more than doubled for that night and Leno has been atop the competition since then.
CBS research chief David Poltrack, who has looked at the late-night audience from all angles, said the "Tonight" show monologue is a powerful tradition that's hard to fight. NBC's show is more popular with news viewers while fewer Letterman fans watch the news at night, he said.
Letterman viewers tend to stay awake longer, for what that's worth.
"There are more people who watch the `Tonight' show and go to bed at midnight than watch the `Late Show' and go to bed at midnight," Poltrack said.
Burnett said he believed Letterman would get a bigger audience than Leno if Dave were on NBC.
"You give NBC executives sodium pentothal and ask (if) they'd rather have Dave or Jay on NBC and see what happens," he said.
NBC executives, albeit different ones, had exactly that choice 11 years ago. They chose Leno.
"You know that lie detector test that Rob Burnett wants me to get under?" Zucker said. "I think that same lie detector test would acknowledge that they have a hard time admitting that America made this decision a long time ago."
Alan Bell, whose Freedom Communications owns five CBS stations, said it's clear that Letterman doesn't have Leno's broad appeal.
"He is never going to have the belly laugh appeal of a Jay Leno. But you can have a very successful show and a good business with David Letterman," Bell said.
One expert on late-night television, Aaron Barnhart, said that since it's clear Letterman is never going to beat Leno, he should stop worrying about it.
Like a politician, Letterman should start framing his legacy, said Barnhart, a Kansas City Star TV critic who once ran a Web site chronicling the late-night competition.
"This was the late-night voice of our generation, the guy who reinvented late-night TV and took it beyond Carson," Barnhart said. "We're really seeing the end of an era and it's closer than people think."
It's been a tough year for Letterman, 56. He missed several weeks because of a painful case of shingles, the ailment coming three years after his heart surgery.
Over the past month, Letterman took Fridays off in favor of guest hosts Tom Arnold, Kelsey Grammer, Jimmy Fallon and Tom Green. It was somewhat odd because it didn't give Letterman an extra day off; his Friday show is usually taped on Thursday. While this was happening, Leno's lead has widened.
The experiment ended after Fallon's show on Friday; Letterman will tape Friday shows for the rest of the summer, a spokesman said.
CBS had no problem with that decision or the ratings: the show is "very profitable" and lends prestige to CBS, network spokesman Chris Ender said.
Burnett said that he realized the competition for ratings "is completely out of our hands."
"We've won five Emmys in a row," he said. "We're doing the best show that we can do. That's all we can control, is to try to keep the show great."
Both "The Late Show" and CBSNews.com are part of the Viacom family.
By David Bauder