Why would Obama supporters do this?

In my city, some vandal spraypainted a bunch of pro-Obama graffiti all over a business building. Also, someone who had a "McCain 08" bumper sticker had his tires slashed... I also heard rumors that many Obama supporters plan to riot if he doesn't win the presidency... Why do many Obama supporters possess such a mentality that provokes them to react violently and resort to illegal activity to get their way?