Debate nights were ruining the chart scale above so I adjusted it. Here are the numbers that were cut off: 2/21 CNN 2,406; 2/26 MSNBC 2,359; 3/4 CNN 1,587.
More Nielsen Cable News Ratings including Viewers Total Day and Primetime, and Adults 25-54 Total Day:

Debate nights were ruining the chart scale above so I adjusted it. Here are the numbers that were cut off: 2/21 CNN 6,023; 2/26 MSNBC 5,592; 3/4 CNN 3,692.

Nielsen TV Ratings Data: ©2008 Nielsen Media Research, Inc. All Rights Reserved.