Jay is really beating Dave in the latest round of the late- night wars.

In last month's sweeps, Jay Leno's Tonight Show (search) on NBC crushed David Letterman's Late Show (search) on CBS by more than 60 percent among viewers 18-49 years old -- the widest margin in four years in that advertiser-coveted demographic.

More than 6.3 million viewers watched The Tonight Show last month compared to 4 million viewers for The Late Show -- continuing a trend that's been in place now for almost eight years.

So why has Jay been more popular than Dave for such a long time?

"Leno just has a broader appeal than Letterman," says Tom DeCabia, a TV industry analyst with PHD, a New York-based media buying firm. "Letterman has such a sharper wit compared to Leno. He can get a guest to feel very uncomfortable very quickly.

"That's why Letterman is who he is, but it limits his appeal across-the-board."

Most viewers assume that Letterman's acerbic, intellectual humor mainly appeals to New Yorkers, but the Nielsen ratings prove that theory wrong. In New York, about 299,000 viewers watched Leno last month on Ch. 4, while 197,000 watched Letterman on Ch. 2.

"In terms of the audience, Jay Leno is perceived to be more middle-of-the-road," says Bill Carroll of Katz Media. "Letterman is considered more 'Big City,' while Leno is considered more 'Middle America. '"

The Tonight Show was further boosted last month by a successful sweeps stunt in which Today co-host Katie Couric (search) switched jobs with Leno for a day.

And TV industry insiders suggested that Letterman's ratings may have been hurt because he was absent from work for 34 days prior to sweeps while recovering from a severe case of shingles.

What was not a factor, however, was how well both NBC and CBS promoted their respective shows.

In the past, Letterman felt that CBS didn't promote his show enough while NBC always promoted The Tonight Show throughout prime-time.

But as part of the $31.5 million deal that Letterman signed with CBS last year, the network has revved up its promotion of The Late Show, which should, theoretically, also be getting a boost from CBS' improved prime-time schedule.