I'm an Independent who leans left, and honestly, I probably would have reacted similarly. If someone comes to my door, before I even open it I look out the window. If I know and trust the person, I will toss the door open and invite them in. If I do not know the person, I will open the door only a little, enough that we can see each other and carry on a conversation but not enough that they could see into the room behind me, and I ask what they would like. Being cautious when opening the door to strangers may be paranoid, sure, but there are just enough lunatics in this world for me to feel like it's necessary not to just openly trust strangers.
If they are selling me politics, religion, or products I don't need, I'll generally smile and say no thanks and close the door-- whether or not I agree with their stance on politics, religion, or think whatever product they are selling is fascinating. Very few people are likely to change their mind based on a stranger harassing them in their homes. I feel that it's a waste of both my time and the stranger's time to allow them to sit there and try to sell me a product or idea when I already know what I'm doing and have everything I need. I'm sorry, I know that sounds very rude and strangers knocking on doors are people too. But I've never met someone who actually enjoyed any sort of salesperson coming up to them in their home. It's a situation that can make people resent you whether they agree with you or not. (If they are there for a different reason, like looking for information on my stances instead of trying to sell me theirs, or they are lost and need directions, etc etc, I will of course try to help them out.)
On top of that, people get bombarded with political stuff so much that by the time the presidential election is over, they are sick to death of it. After about a week straight of getting 3+ political pamphlets for state elections in my mailbox every single day, I will be very annoyed when I see another one in there, I give zero darns who it's for. After months of hearing political ads on the radio every time I take a ride in my car, I'd rather drive wherever I'm going in silence, never mind who the ads are for. Add that to people actually coming to my home, and it's a lovely recipe for anger.
Now, is it ridiculous that people were threatening bodily harm to you or yelling at you? Yes, that's pretty awful and unnecessary. :-( I'm sorry that happened to you and you are right, it's not normal behavior. Perhaps they were overly sensitive following Romney's loss, perhaps they felt you were there to gloat. Either way, simply saying no thanks and shutting the door would have been more appropriate. But this isn't limited to republicans. I'm sure that if the tables were 100% turned-- that is, Romney had won, and republicans were going to the houses of democrats to try to persuade them to vote for other republicans, similar things would happen. It's kinda like rubbing salt in their wounds, I suppose.