It's the cold calculus of the high-stakes world of U.S. network television news -- if your show's ratings go down, your prospects go down with them. And with ABC nipping at NBC's heels, the executive producer of NBC's Nightly News with Brian Williams may pay the price.

An excerpt from the NYT story:

NBC’s dominance in television’s evening news race is undergoing its most serious challenge in a decade as “World News” on ABC scored its second ratings victory in the last three weeks. The figures highlight the slow but steady ascent of the veteran ABC newsman Charles Gibson toward the top position among news anchors.

In what is being widely interpreted as fallout from the shifting ratings picture, NBC has made plans to replace the executive producer of the network’s “Nightly News With Brian Williams,” according to several NBC executives.

A successor to the current executive producer, John Reiss, may be announced as early as today, the NBC executives said, emphasizing that a change in the newscast had been in the works for several weeks and was not related to the most recent ratings results. The NBC executives asked not to be identified because the change had not yet been formally announced.

The looming change in the control room at “Nightly News” and the ratings surge by Mr. Gibson are but the latest developments in the most tumultuous two years in the recent history of broadcast news.

Mr. Williams succeeded Tom Brokaw in December 2004. After that, Dan Rather resigned as anchor at CBS in the midst of a reporting scandal, and was soon succeeded by Katie Couric. Peter Jennings died of lung cancer while still the lead anchor at ABC, and one of Mr. Jennings’s designated successors, Bob Woodruff, nearly died in a roadside bomb explosion in Iraq, setting off a sequence of events that ultimately led to Mr. Gibson’s move to the anchor desk at “World News.”

Only six months before Mr. Gibson got the evening news job, he was effectively passed over for it, in favor of two much younger journalists, making his current run at Mr. Williams’s broadcast all the more remarkable.

Even as some young viewers forsake television for the Internet, the three network newscasts continue to attract a collective audience of nearly 25 million viewers most nights. The programs are also among the most lucrative for the networks, with advertisers planning to spend nearly half a billion dollars on them this year.

More than a few viewers, and advertisers, choose their broadcast based on the personal strengths of those anchors. That has left some people at the networks wondering whether Mr. Gibson — who, at 63, is the oldest and most established of the three — may be proving more attractive to more viewers than Mr. Williams and Ms. Couric.