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.

Mr. Griffin said that Ms. Maddow’s advantages included her regular appearances on “Countdown” and her popularity on the Internet, where, he said, “word spread like wildfire” about her new show. Ms. Maddow, a former AIDS activist, was also presumably helped by her four years on the Air America radio network.

Ms. Maddow and every other cable news anchor are beneficiaries of the heightened interest in the presidential campaign. Fox News Channel’s “O’Reilly Factor,” the highest-rated hour on cable news, reached an average of four million viewers in September; it had two million during the same period a year ago.

Still, Ms. Maddow’s ascent is unique in its swiftness. Her program immediately drew almost half a million viewers ages 25 to 54 in a slot where a quarter of a million is more common. Even if her ratings decline after the election — and history suggests they are likely to — Mr. Griffin contended that Ms. Maddow’s performance confirmed that cable news was “a three-way race now.”

Fox News did not comment for this article. Ryan Jimenez, a spokesman for “Larry King Live,” said in a statement that the shows couldn’t be more different: “While our competitors have moved to partisan extremes, we continue landing the biggest guests because we embrace the vast middle and have a wider appeal,” he said.

The daily rhythm of television ratings has required an adjustment for Ms. Maddow, whose broadcasting skills were fine-tuned on the radio.

“You don’t get overnights in radio,” she said in an interview, discussing the ratings. “This instant 4 p.m. burst of feedback is hard to get used to.”

But she has adopted the vocabulary quickly, praising Mr. Olbermann for a “931 in the demo” last Tuesday. (That night, Mr. Olbermann’s average of 931,000 viewers 25 to 54 lifted MSNBC to a rare prime-time win over both Fox News and CNN.)

Ms. Maddow acknowledges that much of her success can be attributed to the lead-in from “Countdown,” which continues to be MSNBC’s marquee program. For years Mr. Olbermann, a vocal critic of the Bush administration, had pushed the network to install a thematically similar program in the 9 p.m. hour, and in August MSNBC decided to replace “Verdict With Dan Abrams” with Ms. Maddow. While Mr. Abrams on occasion bested Mr. King in the ratings, Ms. Maddow’s wins are coming at a more frequent rate.

In her first six weeks Ms. Maddow on many nights is retaining more than 90 percent of Mr. Olbermann’s audience, a figure that many television executives would envy.

Encouraging the audience to stay put, the two programs often cover similar subjects most nights: on Friday, for example, Ms. Maddow appeared on “Countdown” to discuss the political maneuverings inherent in the contentions of voter registration fraud, and in her following hour she devoted two segments to the same subject.

While Mr. Olbermann watches Mr. O’Reilly’s show on a monitor embedded in his desk, Ms. Maddow insists that she has never watched either Mr. King’s program or the 9 p.m. program on Fox News, “Hannity & Colmes,” which garners more viewers than either of the other shows.

Partly, she said, that lack of competitive interest is an effort to remain original. “I worry every day about the homogenizing forces at work in my professional life,” she said, adding that it can be difficult to preserve creativity within cable’s production process. It helps, she said, that she does not own a television at home.

Even so, Ms. Maddow said, she has finally committed to getting a set, primarily so that her companion can watch her program. With Ms. Maddow delivering MSNBC a record audience, it might seem that the least the network could do would be to deliver her a television.