It's all very simple--they're trying to attack McCain's strength. Hell, it worked for McCain in attacking the Obamessiah as an agent of "change," so it should work the other way, should it not?
The one fly in the ointment is this--McCain's demonstrated that he has courage, and with his pick of the Sarahnator as his running mate, he shows that he can lay a legitimate claim to the "change" mantle. Obama has voted with his party 97% of the time in the Senate--showing that he's more liberal than either Hillary Clinton or Teddy Kennedy--and has sponsored exactly one bill with a Republican co-sponsor, and it was a re-authorization of an act that was passed with bipartisan support in 1996.
One is the product of, arguably, the most corrupt political machine in the United States--Chicago's Democrats--and the other placed on his ticket someone who ran against arguably the most corrupt Republican political machine in the United States--Alaska--and won. Who's the agent of "Change" ehre?