Take the line "George W. Bush is an idiot" and precede it with the declaration of either "I am a Republican" or "I am a Democrat" to make a compound sentence. What conjunction would you choose to combine the clauses?
For the elephant, likely it would have to be "I am a Republican but George W. Bush is an idiot."
For the donkey, it might go "I am a Democrat and George W. Bush is an idiot."
(If you used "or" then you're just being silly.)
The conjunction does not merely put the two together; it suggests shame or glee. One little variation and the compound sentence takes on completely opposing tones.
Language is neat.