A day after CNN announced that it was hiring Campbell Brown to replace one of its prime-time hosts, presumably Paula Zahn, Ms. Zahn confirmed today that she was leaving the cable channel, effective Aug. 2.

The unraveling of “Paula Zahn Now,” which made its debut at 8 p.m. in spring 2003, was ultimately a function of ratings. Though CNN took pains recently to note that the number of viewers for the show had ticked upward earlier this year, Ms. Zahn’s task remained a Herculean one.

The estimated 558,000 viewers her program has been drawing, on average, each weeknight this year, according to Nielsen Media Research, represents less than a quarter of the nearly 2.3 million who watch “The O’Reilly Factor” with Bill O’Reilly on Fox News. Ms. Zahn’s program also draws about 100,000 fewer viewers a night than “Countdown with Keith Olbermann” on MSNBC.

O'Reilly has been losing viewership, particularly in the key 25-54 demographic so it was not O'Reilly that did Zahn in. It was Olbermann, whose Countdown program is now the MSNBC flagship and ratings driver. I do wonder what CNN thinks Campbell Brown can do for them in the time slot. Mrs. Dan Senor (of Iraqi CPA fame) will, one assumes, attempt to do better in the 25-54 demo. But how she steals from Olbermann is hard to see. I guess she will be going after O'Reilly to get younger conservative viewers. We'll see.