There's not even agreement on whether watching Stewart and Colbert is positively correlated with youth political participation in itself, much less partisan identification.
http://apr.sagepub.com/content/34/3/341http://ijpor.oxfordjournals.org/content/20/1/90http://www.tandfonline.com/doi/abs/10.1080/10584600802427013#.U2kVJsZ6elIOne of these articles found that both Bush and Kerry (outdated, yes) were viewed more negatively among young viewers of these shows, even controlling for ideology. It seems that if these Comedy Central programs are indeed "selling the Democratic Party," it's a fairly recent phenomenon.
