Some people feel so strongly about one party, they're willing to put up a poster in their window telling everyone who they're backing. Now these are very persuasive, aren't they? Oh look, that person who lives opposite whom I've never even said hello to is a Labour supporter. Ooo, I'd better vote the same way they do.
Thankfully the majority of people don't nail their colours to the mast (or window) in such a blatant way. Probably because most of them couldn't give a you know what.