Basically, he's using humor (maybe it doesn't translate well) to describe a horrible situation, a disaster. Lice are little insects, like fleas (those little bugs that make your dog itch and scratch). So that's not a nice thing. And neither are rats. So lice + rats is already bad. A corpse is a dead animal. Also not good. So one disgusting thing (lice) on another horrible thing (rats) on another horrible thing (dead horse)... and the horse is on fire, it is burning, like dead horse BBQ. Pretty horrible as well. It's a pile of ugly metaphors: add them up, and you have a disaster.

Hi Yeoul, Even as a 51-year old American that phrase confuses me. It seems to be an internet meme, so maybe I need to be twenty years younger. I think I can explain the "lice on rats." Lice are tiny pests (insects) that people can get in their hair and make them itch. Rats are larger pests (mammals) that people can get in their home. So he seems to be comparing the election to a disgusting pest (a rat) that is covered in another kind of disgusting pest (lice). Someone else will have to explain the "horse corpse on fire". Also I'm not sure what you mean when you describe the election as incorrigible--that adjective is usually applied to people, usually to say they have a bad quality that won't change. Example: "He's an incorrigible liar."