At the end of a month in which the midterm elections became a dramatic national news story, cable-news underdog MSNBC registered faster audience growth than the competition, helped by “Countdown With Keith Olbermann” and its late-night slate of taped news features.

The fact that the network started with a smaller audience than No. 1 Fox News Channel and second-place CNN made those percentage gains easier to post. But the 8 p.m. “Olbermann” increased its total audience 66 percent to 744,000 in November, compared with a year earlier. MSNBC’s 10 p.m. to 11 p.m. documentary slate drew a 31 percent bigger audience than last year’s live shows, helping to vindicate a strategy that was derided by news purists when it kicked off in the fall.

Among the 25- to 54-year old viewers most prized by advertisers on news programs, MSNBC’s “Olbermann” still finished a distant second in the 8 p.m. to 9 p.m. hour behind its favorite on-air nemesis, Fox’s “O’Reilly Factor.” The hour that kicks off the news channels’ lucrative prime-time blocks closed the month with “O’Reilly” averaging 2.12 million total viewers, down 17 percent. At CNN, “Paula Zahn Now,” long the most vulnerable show on CNN’s evening lineup, averaged 766,000 total viewers, up 6 percent.

In the key news demographic of 25- to 54-year-olds, “Olbermann’s” audience almost doubled to 304,000. “O’Reilly” attracted 501,000 in the age group prized by news-show advertisers, posting a 7 percent gain. “Zahn’s” audience in the demographic rose 46 percent to 265,000.

In prime time for the month, Fox’s average total audience fell 19 percent to 1.35 million total viewers. CNN posted a 15 percent prime-time gain to average 826,000, and MSNBC’s average audience grew 29 percent to 505,000.

Among viewers in the demographic, Fox gained 10 percent during prime time to attract 380,000 viewers, while CNN gained 40 percent to 271,000 and MSNBC increased its audience 38 percent to 210,000.

For total day, Fox averaged 755,000 total viewers (down 13 percent) and 257,000 in the demo (up 13 percent). CNN averaged 515,000 total viewers (up 15 percent) and 271,000 (up 40 percent) in the demo. MSNBC averaged 320,000 total viewers (up 25 percent) and 139,000 in the demo (up 43 percent).

While liberal opinionated ranting helped “Olbermann,” more conservative jabs appeared to pump up the audience for CNN veteran “Lou Dobbs Tonight” in the 6 p.m.-7 p.m. slot. The program grew 39 percent in total viewers to 962,000, and 73 percent in the demographic, posting an average of 300,000 for the month.