NEW YORK (CNN/Money) - When George Bush and John Kerry square off for the second time Friday night, top network executives will be listening to a giant sucking sound.

Round 2 is next

It happens every four years. Presidential contenders verbally joust on primetime television and the networks airing 90 minutes of uninterrupted coverage lose tens of millions of dollars in advertising.

This time around, independent research analyst Jack Myers estimated that the four top networks -- CBS, NBC, ABC and Fox -- are collectively losing anywhere from $40 million to $50 million on each debate.

With three Bush v. Kerry face-offs this fall, that comes to $120 million to $150 million for this election cycle.

That does not count another wallop from Tuesday night's vice-presidential debate, which Fox did not air due to the baseball playoffs. Nor does it include the advertising losses suffered by the three cable networks, specifically MSNBC, Fox News, and CNN (a corporate cousin of CNN/Money).

"Do they recoup it? No," said Bill Carroll, the director of programming at Katz Television Group.

For networks, election-year debates have long been part of the trade-off of doing business. In exchange for government licenses, they broadcast programming deemed to be in the public interest. While the national party conventions, more circus than serious, no longer qualify, debates do.

Click to see how much the candidates have raised and more on the election.

Networks have long factored the losses from presidential debates into their projections. And it helped this year that round one of Bush v. Kerry drew 62.5 million viewers, more than any national debate since 1992. The vice-presidential debate, with 43.6 million watchers, was also the biggest draw for a vice-presidential debate in 12 years.

There is some payoff for the networks. Like the Olympics, the debates attract irregular viewers. Some stick around to watch the post-debate analysis, local news and late night shows.

Network news divisions benefit from being showcased. Although they intensify the financial pain with lavish productions, channels that draw the most viewers, like NBC did on both debate nights, win bragging rights.

More network news • Shows voters watch • Fall TV: Reality bites • CBS socked $550K • Fox to Ivana: "You're hired!"

"All of this is positioning," said Carroll. It also gives broadcast networks a chance to compete with 24/7 cable news. "The networks are trying to say 'we're not going to cede this exclusively to the cable networks.'"

Still, without the commercials, there's relatively little for network mercenaries to cheer when Bush and Kerry duke it out.

Debate broadcasts have "more negative implications than positive," said Myers, publisher of the daily Jack Myers Report.

Over time, the downside is becoming increasingly apparent.

NBC, CBS, ABC and Fox are all struggling to stop a steady decline in ratings as viewers shift to cable, the Internet or other forms of entertainment. Lower ratings weakens their ability to draw advertisers at top rates.

Less than a month into the primetime television season, networks are still in seduction mode with their new shows. Now, with the debates, "they're losing momentum on four nights at the beginning of their season," said Carroll.

The Commission on Presidential Debates, a nonprofit that organizes these matchups, could not have picked a worse night to host last week's inaugural debate between Kerry and Bush.

Thursday night has long been the networks' biggest revenue generator of the week, helped by movie studios and automobile makers eager to lure the weekend crowds. "All the advertisers want to be there," said Andrew Donchin, director of national broadcast for media buying firm Carat USA.

YOUR E-MAIL ALERTS John F. Kerry John Edwards George W. Bush Dick Cheney or Create your own Manage alerts | What is this?

Friday night, when lower advertising rates reflect a predictable drop in viewers, is better. But Wednesday, the day of the final debate on Oct. 13 and traditionally another big advertising night, will be another hit.

"Networks would have no problem putting the debates on a Saturday because nobody watches network TV anymore on that day," said Donchin. "But the debates are something they have to cover. It's just something they do."