“The explosion of video-sharing on the Web poses major risks for presidential candidates: Gaffes and inconsistent statements witnessed by dozens can be e-mailed instantly to millions ...The impact of a negative video can be devastating — and undetectable.” L.A.Times.
The L.A.Times has an excellent article (Attack ads go online and underground) this week discussing how political candidates make use of web-videos as a campaign tactic. The same topic is also discussed in ABCnews’ article Blogging Going Wild: Open Government or Gotcha Politics. I would strongly recommend you to read the two articles, as both have an insightful take on a relevant topic!
Update: The NewYork Times article Link by Link - In Politics, the Camera Never Blinks (or Nods) is also discusses this.
and more: Technology Evangelist - The Internet's Effect on 2008 Presidential Campaign and Denver Post - Politics tuning TV out are also discussing the topic.