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Phil, I think there's a fair degree of tribalism on the lefty side. It's not confined to UK politics either. While delivering a campaign speech one day Theodore Roosevelt was interrupted by a heckler: "I'm a Democrat!" the man shouted.
"May I ask the gentleman," Roosevelt replied, quieting the crowd, "why he is a Democrat?"
"My grandfather was a Democrat," the man replied, "my father was a Democrat and I am a Democrat."
"My friend," Roosevelt interjected, moving in for the kill, "suppose your grandfather had been a jackass and your father was a jackass. What would you then be?"
Alas, Roosevelt was thwarted by the quick-witted heckler, who promptly replied: "A Republican!"