NEW YORK — CNN is poised to finish March third in the prime-time weeknight ratings behind Fox News Channel and MSNBC, the first time this has ever happened for the channel that pioneered the cable news genre nearly three decades ago.

CNN says its overall business is healthy and it is not straying from its straight news path. But it is suffering more audience erosion than its rivals since the peak days of the presidential election, further proof that the opinionated prime-time shows on Fox and MSNBC have greater audience loyalty.

CNN's weekday prime-time ratings are relatively flat compared to last year during the primary campaign, up 1 percent from March 2008, according to Nielsen Media Research. Fox's ratings have jumped 30 percent and MSNBC, the new No. 2, is up 24 percent. The biggest growth in cable news is for CNN's partner, HLN, formerly Headline News, which is up 62 percent.

Fox remains on a mountain above its two closest competitors, with its prime-time audience in March more than that of MSNBC and CNN combined. "The O'Reilly Factor" has done particularly well, keeping more of its postelection audience than anything else on CNN and MSNBC.

Through Wednesday, Fox was averaging 2.73 million prime-time viewers in March. MSNBC had 1.16 million and CNN had 1.14 million. The March ratings period ends Friday, and it's doubtful CNN will be able to overcome MSNBC.

"The fact that one network may have eked out a slight edge in one small slice of the overall business really doesn't say much of anything," Jon Klein, CNN U.S. president, said on Friday. "It's more clear than ever, given the way that our competitors have positioned themselves, that CNN has positioned itself as the real news network."

Relying on news, rather than opinion, leaves CNN more susceptible to higher ratings peaks during big stories and lower valleys in routine times. Yet it's hard to consider the present — new president, economic turmoil and two wars — a slow news period.

CNN's ratings news "is very significant," said Frank Sesno, a former CNN Washington bureau chief and now a professor at George Washington University. "This is a big problem."

More significant is what CNN's ratings problems mean coupled with the daily drumbeat of layoffs in the newspaper industry, he said. With people more interested in hearing things through an ideological prism as a form of entertainment, it diminishes the value of independent voices giving straight news.

"It's getting harder to do real journalism on television," Sesno said. "This is 'man the ideological barricades.'"

Fox is ready to start a new venture Monday, "The Fox Nation," which it bills as an online community that believes in "your right to express your views, your values, your voice." Fox representatives would not immediately return a call for comment.

The most problematic part of CNN's prime-time schedule is Campbell Brown's 8 p.m. show, up against O'Reilly and MSNBC's Keith Olbermann. Brown's audience is smaller than any prime-time show on the three networks, and beneath Nancy Grace's crime hour on Headline News.

Brown leaves for a six-to-eight week maternity leave following Friday's show, and will be replaced temporarily by Roland Martin. Klein said Brown's show isn't in any danger, noting that it took years for Olbermann and O'Reilly to build their audiences and Brown has been in her job for a year.

There's been no talk of moving Grace to CNN, he said. Having Grace's crime-oriented show on Headline News allows CNN to keep its focus on being a news network, he said.

MSNBC's Rachel Maddow is a close third to Larry King, and both are beaten handily by Sean Hannity's new Fox solo show. At the 10 p.m. hour, a rerun of the show Olbermann did two hours earlier has been doing surprisingly well against CNN's "Anderson Cooper 360," leading MSNBC to at least temporarily put on hold any development of a new live show then.

MSNBC chief executive Phil Griffin said the prime-time ratings are an affirmation of the network's decision to go liberal with Olbermann and Maddow. But he also said it pointed to problems at his rival.

"They've got the best brand in news," he said. "CNN, that's better than anybody. But you've got to deliver on that — and they're not. It's a hollow promise."

He compared CNN to ESPN, which started at the same time, saying that, while ESPN has evolved aggressively and remained the leader in sports, CNN hasn't. Their evening lineup of Lou Dobbs, Brown, King and Cooper lacks any consistency or flow, he said.

"What do they stand for?" he said. "That's their biggest challenge. CNN ain't what it used to be, and that has given us an opening because we stand for something and they don't."

Klein dismissed Griffin, noting CNN is beating MSNBC handily when the full day — not just prime-time — is taken into account. He said that for March and the year's first three months, it was CNN's best showing since 2003, when the Iraq War started.

CNN continues to have a greater reach and reputation than its rivals across all platforms, he said. When you measure viewership across the full day — not just prime time — CNN has a comfortable lead over MSNBC.

"When you have other so-called news networks ceding the field of journalism, we are happy to fill that void," Klein said. "It's working for us."