According to Derek Willis of The Upshot of The New York Times :
The parade of politicians on the Sunday morning talk shows veers to the right, not the left.
Conservative members of the current Congress have appeared more often on the network talk shows than their liberal counterparts. Senators and representatives from the conservative end of the ideological spectrum have made 57 percent of the appearances, compared with 42 percent for liberals, according to an Upshot analysis of data collected by American University.
http://www.nytimes.com/...
Furthermore, Willis notes that 56% of the appearances by former members of Congress are by conservatives.
Although Willis does note that the frequency of appearances by some conservative elected officials, such as John McCain, explains the difference, I believe that this does not justify the lack of balance. There are many other liberal members of Congress (such as Elizabeth Warren) who could be asked to appear more frequently to give the programs more balance.
It appears to me that the Republican spin that the media has a liberal bias has been successful as the people who book guests on the programs have either consciously or unconsciously scheduled more Republicans to avoid this bias.
I hope this article will be read by members of the media and that they will make a better effort to create more balance on their programs.