Redefining the "Undecided" vote as the "Leave me the **** alone" vote.
I suspect most so-called Undecideds have really already decided but are annoyed at the intrusion.
OTOH, some may enjoy the attention connected with Undecided status.
A third group: I know people who actively lie to pollsters just out of cussedness. They aren't rude to individual pollsters, but they treat polls as just another form of telemarketing.
