A derogatory term Republicans use to refer to the Democratic Party. This term comes from the fact that the Democratic Party's mascot is the Donkey, and the fact that "Jackass" is another name for "Donkey".
The thought process goes like this:
Democratic Party = Donkey = Jackass ---> Jackass Party
"Jackass Party" has a two- fold meaning. On one level it refers to the Democratic Party's mascot- the Donkey. On another level it expresses the common Republican view that Democrats are Jackasses - hence the "Jackass Party" (or the party of jackasses who's mascot is a jackass).
GUY ON THE STREET: Can you believe what Obama just did?! What a bad idea!
REPUBLICAN: That's what happens when you have a member of the Jackass Party running things.
DEMOCRAT: I love ObamaCare, and marijuana, and political correctness, and Hillary Clinton, and Planned Parenthood, and welfare, and government programs.
REPUBLICAN: So. You're a member of the Jackass Party, huh?
A gathering or cluster of jackasses engaging in stupid behavior and/or behaving like an ass.
The traffic jam around the school was one big jackass party.