OrangeHoof suggested that true information should be added, the simple fact that they are Democrats. Is that misinforming or defacing?
Perhaps a fair compromise would be to put the bumper stickers on a separate sign next to the candidates.
To be honest, I’m attaching the candidate to Obama, not the Democrat Party. They may not like that but is that any different than attaching Republicans in 2008 to George W. Bush? The Dems routinely did that to GOP candidates then.