Rachel Maddow, a woman who does not own a television set, has done something that is virtually unheard of: she has doubled the audience for a cable news channel’s 9 p.m. hour in a matter of days.

More important for her bosses at MSNBC is that “The Rachel Maddow Show,” her left-leaning news and commentary program, has averaged a higher rating among 25- to 54-year-olds than “Larry King Live” on CNN for 13 of the 25 nights she has been host. While the average total audience of her program remains slightly smaller than that of Mr. King’s, Ms. Maddow, 35, has made MSNBC competitive in that time slot for the first time in a decade. The channel at that hour has an average viewership of 1.7 million since she started on Sept. 8, compared with 800,000 before.

Given that advertising dollars  and the reputations of networks  rise and fall on prime-time ratings, Ms. Maddow’s rise has been closely watched by media executives.

“I’m pinching myself,” said Phil Griffin, the president of MSNBC, who used to caution that it “takes two or three years for a show to find its audience.” That was certainly true for Keith Olbermann, whose five-year-old “Countdown” program at 8 p.m. (which leads into Ms. Maddow’s program) now beats CNN in the 25-to-54-year-old demographic segment every evening.