This joke as I originally heard it was significantly more offensive (the setting was Mississippi, rather than the fictional Forum-land, and the joke invented circa 1950 or so...), so I made up a few details out of pure cloth to change it...

A fellow in Forum-land went to the dentist and explained to Big Dentist, DM, that he needed a tooth pulled.

"are you liberal or conservative?" Big Dentist, DM, asked the fellow, and he answered "Liberal".

"Well, that'll be $75, and I'll need to schedule you for an hour appointment--how will that work?"

The fellow said that it would work just fine, and then explained that he had a friend who also needed a tooth pulled.

"Is this guy conservative or liberal?" Big Dentist, DM, asked, and the fellow answered, "Well, he's a conservative."

"Oooh... Well, it's going to be $1,800 and he'll need to stay three days."

"I'm probably going to wish I hadn't asked," the fellow started, "but why the price difference and the stay?"

"Well, they're not allowed to open their mouths around here, so we have to go in through the bottom."