CNN, Fox News, and MSNBC, the three largest American cable news networks, are winning viewers and advertising from their increased coverage of the presidential election campaigns.

The cable news networks more than doubled their ratings for the Democratic convention in July compared with 2000, according to Nielsen Media Research.

Broadcast networks ABC, CBS, and NBC lost a total of 18.5% of viewers, the research company said.

The broadcasters each said they plan to televise three hours of the four-day Republican convention this week, the same amount they did for the Democratic convention in July.

“News viewers have shifted to cable from broadcast news,” said Fox News advertising chief Paul Rittenberg. The trend may be “unstoppable,” he said.

The cable channels will probably attract 20% of the $600 million spent on political ads this year, said Evan Tracey, president of Campaign Media Analysis Group, which tracks such expenditures.

Cable is benefiting from more political ad spending by independent groups such as MoveOn.org and Swift Boat Veterans for Truth, Mr. Tracey said.

About $230 million has been spent on presidential campaign ads through the first week of August, with another $370 million in projected spending before the November 2 election, Mr. Tracey said.

In the 2000 election,$200 million was spent on political advertising, he said.

News Corp.’s Fox News has seen an eightfold increase in spending by President Bush and his Democratic challenger, John Kerry, compared with four years ago, Mr. Rittenberg said. He declined to specify how much the network has collected this year for political ads.

Political ad revenue at Time Warner Inc.’s CNN this year has surpassed the network’s total in the last presidential campaign, said Greg D’Alba, the cable channel’s head of advertising sales.

Mr. D’Alba declined to provide specific ad-revenue figures. He said the network has aired more ads from groups outside the campaigns, such as MoveOn.org, a group against Mr. Bush, and Swift Boat Veterans for Truth, whose ads have been critical of Mr. Kerry’s war record.

General Electric Co.’s MSNBC expects campaign-ad spending to exceed $1 million this year, said Seth Winter, vice president for advertising sales for network news and MSNBC.

“We received no national, presidential campaign money last go-around,” Mr. Winter said. “We’re seeing very healthy spending now” from both campaigns as well as independent groups, he said.

The gain may boost sales at the cable-news channels’ parent companies.

Revenue at CNN and Time Warner’s other cable networks, which contributed 22% of the company’s sales for the first six months of 2004, rose 7.7% to $4.57 billion during that period.

Cable-network sales at News Corp. rose 11% to $2.54 billion for its fiscal year ended June 30.The unit, which includes 12 networks such as Fox News and the FX channel, provides about 12% of News Corp.’s revenue.

Revenue at General Electric Co.’s NBC Universal division, which runs NBC and cable channels such as MSNBC, rose 30% to $4.45 billion during the first half of the year.

About three-quarters of political spending this year will be directed to local TV stations, said Campaign Media’s Mr. Tracey. That’s benefiting companies that own stations, including network owners such as Viacom Inc. and smaller operators such as LIN TV Corp., which runs 23 stations.

Viacom’s CBS stations group, which runs 16 TV stations such as WCBS in New York, increased second-quarter advertising sales 8%, helped by campaign commercials,Viacom said in July. Sales at Providence, R.I.-based LIN TV rose 8% in the second quarter to $96.3 million, with about one-third of the gain stemming from political ads, the company said.

MoveOn has spent about $17.2 million on advertising, with about 5% devoted to cable-news networks, said Bill Zimmerman, who’s managing advertising for the organization. MoveOn has gotten some funding from billionaire investor George Soros, who has given $12.6 million to MoveOn and other anti-Bush groups as of June 30, according to election records.

The Swift Boat group has spent about $1.3 million on Kerry ads, about $30,000 of that on cable television, as of August 25, spokesman Sean McCabe said.

More than half of Americans have seen or heard about the Swift Boat ad because it received coverage on cable news and radio talk shows, according to the University of Pennsylvania’s National Annenberg Election Survey.

The three cable-news networks broadcast three and a half hours of Republican convention coverage on Monday night while ABC, CBS, and NBC aired entertainment programs. Fox News drew the biggest audience, with 3.9 million viewers, while CNN pulled 1.3 million people and MSNBC attracted 854,000 viewers, Nielsen said.

CNN, Fox, and MSNBC had an average of 5.66 million viewers on each of the four nights of the Democratic convention in Boston, up from 2.74 million at the Democratic convention in Los Angeles in 2000.

ABC, CBS, and NBC drew an average of 13.8 million viewers for each night of this year’s convention, down from 17 million four years earlier, according to Nielsen Media Research.

The broadcast networks cut their total coverage to 578 minutes in Boston from 745 minutes in Los Angeles, according to CBS. ABC, CBS, and NBC each plan to broadcast about one hour of live coverage for three of the four nights from the Republican National Convention, the networks said in statements.

The three American broadcast networks have been trimming coverage of conventions because the events lure fewer viewers than entertainment fare, said Susan Nathan, a researcher at Universal McCann, an ad-buying firm in New York.

The audience watching Viacom’s CBS during the last night of the Democratic convention on July 29 dropped to 5.5 million people, less than half than would have tuned in for its regularly scheduled crime-drama “Without a Trace.”

The shift of viewers to cable reflects a trend among television audiences, which are increasingly abandoning broadcast shows for programs such as USA Network’s mystery series “Monk” and the drama “Nip/Tuck” on News Corp.’s F/X network.

Cable-TV channels have increased their audience to 31% of homes with TV sets this season from 24% four years ago, Nielsen said.

“People say they are continuously relying more on cable news” and advertisers are following them, said Stacey Lynn Koerner, a researcher at Initiative Media, a New York-based ad-buying firm.

The broadcast networks are making a mistake by scaling back convention coverage, said CNN anchor Wolf Blitzer, whose nightly program often focuses on politics. Even though the political parties “script literally every second” of their conventions, CNN viewers would be disappointed if the network cut back coverage, he said.

Ads on cable can give national visibility to a political commercial, while buying time on local stations lets campaigners target audiences in contested states.

A 30-second advertisement on “Larry King” cost about $6,700 in June, according to Nielsen Monitor-Plus, which tracks ad spending. A spot on a local television station in Albuquerque, N.M., costs about $750, according to an ad buyer at McKee Wallwork Henderson in Albuquerque, Karen McCallum.

Much of the political advertising is being directed to local TV stations in states where the race is perceived to be the closest such as Ohio and Pennsylvania, said Campaign Media’s Mr. Tracey.

The general manager of family owned NBC station WFMJ in Youngstown, Ohio, John Grdic, said spending between Mr. Kerry, 60, and Mr. Bush, 58, is “neck-to-neck. One week it’s Kerry and we get no orders from Bush, and the next week” it’s reversed, he said.

Mr. Kerry’s campaign has spent about $80 million so far on commercials, with about 5% going to cable, said Kerry spokesman David Wade.

Bush campaign spokesman Scott Stanzel declined to comment on how the campaign is breaking down spending between broadcast and cable.

Bush’s campaign bought three times as much time on cable-television during June as Mr. Kerry, according to Nielsen Monitor-Plus.

Mr. Bush aired 276 commercials on MSNBC, 211 on CNN, and 114 on Fox, while Mr. Kerry had 169 on CNN, 31 on Fox, and 5 on MSNBC, Nielsen said.