I'm personally of the opinion that people who spout such nonsense are perfectly aware that it's nonsense; they're simply trying to appeal to the radicals: the vocal, irrational minority.
The reason to do so is also perfectly rational - they're easier to get to the voting booths to vote for your candidate with such rhetoric. The moderates, who are unswayed (yet annoyed) by such rhetoric are more likely to be the ones who do not show up at the voting table, because they see the rhetoric from both sides as equally reprehensible, and will either not vote at all (in my case, due to the futility of voting in the US's current two party system), or vote for a third party (making their vote equally irrelevant as if they hadn't voted at all).
So, both sides appeal to those folks who are so easily swayed by irrational comments which make them feel superior to "those dirty left/right-wingers", and the result is power (government positions) and money (PACs, lobbyists, donations).
I think a lot of people really do believe their own non-sense. It's easier to lie when one honestly believes the falsehood. Consider that the same instincts that drive partisan politics are the same instincts that drive spectator sports. Are the fans cheering because of a conscious awareness of political strategy? Or are they cheering because -- gosh darnit -- being on a team is fun. Do you think the children in the Robbers Cave Experiment were masterminds of political intrigue? Or do you think the conflict arose out of instinct. That the strategy happens to turn out super-rational in the field is orthogonal to the question of whether their beliefs bubble up to a level of conscious awareness.
I've met people who honestly have made the "dey must be evil" maneuver. My experience says the sentiment is genuine.
The reason to do so is also perfectly rational - they're easier to get to the voting booths to vote for your candidate with such rhetoric. The moderates, who are unswayed (yet annoyed) by such rhetoric are more likely to be the ones who do not show up at the voting table, because they see the rhetoric from both sides as equally reprehensible, and will either not vote at all (in my case, due to the futility of voting in the US's current two party system), or vote for a third party (making their vote equally irrelevant as if they hadn't voted at all).
So, both sides appeal to those folks who are so easily swayed by irrational comments which make them feel superior to "those dirty left/right-wingers", and the result is power (government positions) and money (PACs, lobbyists, donations).