The threats are most likely coming from Romney supporters masquerading as pro-Obama types.
This is an old Black Op tactic that the right in general is fond of.
Try to remember which party broke into the Florida recount centers in 2000 and beat up the employees there. They were Republican staffers organized into goon squads.
Also, try to remember who was making threats and breaking the windows of Democrat headquarter offices after Healthcare reform passed.
Finally, which side has all the armed militias working on its behalf?
Don't be silly. The majority of voters in the ten most violent cities in the nation cast their ballots for Barack Obama in 2008. They didn't get their well earned reputations by having Republicans doing their dastardly deeds.
Armed militias? Most of your Black Bloc member are left wing anarchists who are trained to harass and hurt law enforcement.