MIT media scholar Henry Jenkins (Convergence Culture etc) sums it up nicely in a recent blog post:
As we pass this content along, it facilitates conversations among friends and it allows us to signify to each other our mutual recognition and respect for the civic rituals which surround the political process. When people send me this video, they intend it as a gift -- which is to say, they intend it to reaffirm the social ties we feel towards each other.
Politics - as much as any other part of culture - always had an important social dimension, but the ease with which it is now possible to share and reconceptualise media content is elevating the process to another level.
Also, an attack on the term "viral":
If I had my way, the term and "memes" along with it would be retired from our vocabulary of talking about how media circulates. There's something sick and unhealthy about the concept of viral media. The term, "viral" operates off a metaphor of infection, assuming that the public are unwilling carriers of messages -- yet I doubt very much that the students who sent me this video were in any sense unwilling or unknowing about what they were doing. The concept of "viral media" strips aside the agency of the participants who are sending along this video for their own reasons -- in this case, a mixture of political zeal and personal affection and probably some sense that I would find the video intellectually interesting.
Personally I wouldn't worry too much about that. I'm not committed to the metaphor, but I do believe there is a virus-like quality to a certain kind of cultural content. It is that urge that makes it irresistable to share it with friends. And yes, quite often that might be unknowingly done. Jenkins proposes "spreadable media", which really could be anything from papyrus rolls to radio.
Also, introducing the MIT Center for Future Civic Media!
Also, a funny clip I hadn't seen before: McCain REALLY is the Penguin!