If anything could slow the inexorable decline of evening news viewership, it should have been this year's riveting presidential campaign, which produced a ravenous appetite for political news.

The cable news networks benefited mightily during the race, more than doubling their prime-time audiences this fall. And some broadcast news programs saw big gains: "60 Minutes," up 12 percent this season, attracted 25 million viewers last Sunday for its interview with President-elect Barack Obama and the future first lady, Michelle Obama, the largest audience for a television program this season.

But the overall number of people watching the network evening newscasts actually shrank slightly in the last months of the presidential campaign despite the programs' heavy political focus -- down 280,000 viewers since Sept. 22 compared with the same period last year, according to an examination of Nielsen Media Research data.

Of the three programs, the top-rated "NBC Nightly News" was the only one to register an uptick in audience, growing 1 percent to 8.37 million viewers. ABC's "World News" fell 2 percent, to 8.2 million viewers, while "CBS Evening News" dropped 3 percent, to 6.1 million viewers.

The contraction of the audience at a time when viewers aggressively were seeking news speaks to the challenge the programs face in a landscape increasingly cluttered with media choices.

Once the dominant news platforms in the United States, the three flagship network newscasts found themselves jockeying this season with cable channels and Internet sites to deliver the freshest political updates.

"There was just a lot of coverage out there, and with the 24/7 nature of the cable networks, you could hear whatever you wanted to hear at any time," said Andy Donchin, director of media investment for the ad-buying firm Carat. "With all the choices and options, continual erosion is inevitable."

News executives argued that the relatively small drop-off in viewers this season actually spoke to the lasting strength of the 30-minute broadcasts.

"The fact that, given the competitive environment, we're holding our audience over last year is significant," said Jon Banner, executive producer of "World News." "I don't think we get the big bumps that you've seen in some other media, but we don't get some of the big drops they're getting now."

The cable news networks -- which challenged the broadcast networks' dominance on many big political nights this year -- saw their prime-time viewership fall substantially during the first week after the election. (However, all three had higher ratings than the same week last year.)

There's no question that cable news saw the most momentum during the campaign. NBC executives credited exposure on sister cable network MSNBC with helping the evening newscast's 8 percent gain among viewers ages 25 to 54, those most coveted by advertisers.

This year's campaign was supposed to serve as a showcase for the network anchors, all of whom were making their presidential-election debut.

But Brian Williams, Charles Gibson and Katie Couric did not dominate the 2008 race the way their predecessors did in past elections.

The anchor who attracted the most buzz was Couric, who conducted a series of much-talked-about interviews with Alaska Gov. Sarah Palin. For all the attention, however -- including a parody on "Saturday Night Live" -- "CBS Evening News" still suffered the biggest drop in viewership.

"I don't have an explanation," said Sean McManus, president of CBS News. "I think we did a really, really good job, and I would be lying if I didn't say the ratings were disappointing."

Part of the challenge is the time slot, he said, "an increasingly more difficult time period for people to sit down in front of their television sets."

It has been a chronic issue as American lifestyles have changed. Since 1991, the combined audience of the three broadcasts has dropped from 36.7 million to 22.7 million.