During the three most lucrative and widely watched hours of TV, the cable news channel MSNBC outranked CNN for the first time ever in March.

Viewers continued to flock to the TV versions of the op-ed page in the evenings, according to Nielsen Media Research ratings released Tuesday, posing trouble for CNN, which placed third for the month in weekday prime time from 8 to 11 p.m.

Fox News Channel, the longtime No. 1 cable news operation and a unit of the News Corporation, has extended its lead over CNN and MSNBC in recent months. In March, “The O’Reilly Factor,” Fox’s 8 p.m. program, reached a milestone of 100 consecutive months as the most popular program on cable news.

Mr. O’Reilly’s wide audience  an average of 3.4 million each night  reaffirms the popularity of fiercely partisan programs on cable, especially in the evening. During the prime time hours of 8 to 11 p.m., when the right-leaning Mr. O’Reilly and the stalwart conservative Sean Hannity are hosts of shows on Fox News, the network is the second-most-popular channel on ad-supported cable, with an average of 2.2 million viewers. Only the drama-oriented channel USA averages more viewers.