What's a blue slip?
"What's a blue slip?"
If two senators from a given state (I think two is required now- at various times it's been only one required) object to a judge from their state, that judge never sees the light of the Judiciary Committee. A senator's making that objection is called using the "blue slip."
It's a real problem in places like California or Michigan with 2 democratic Senators. Bush really has to curry favor with them in advance and he'll usually have to nominate more moderate Republicans than he would otherwise.