By RICK KLEIN Your Thursday transition agenda in transition: One president leaves the stage (his last public appearance before leaving office, in a primetime address), while another gets ready to assume it. Sen. Joe Biden and Sen. Hillary Rodham Clinton say goodbye to the Senate, while Roland Burris and Ted Kaufman say hello. The Obamas move into their new new home, closer to that other one they’ll be in soon. A Senate vote — plus a related House vote — on a financial bailout package that’s jointly owned by the outgoing and incoming presidents. And the Republican Party makes its slide into a full-fledged opposition party. If you track no other set of confirmation hearings this month, pay attention to Eric Holder’s appearance before the Senate Judiciary Committee, starting at 9:30 am ET Thursday. It’s not that Holder isn’t going to be confirmed as attorney general. It’s not that we’re likely to learn substantially more about Holder or his background, or even the broader and critical topic of how President-elect Barack Obama’s administration intends to lead the Justice Department. It’s that this is the nomination battle Republicans have chosen to have. (If it wasn’t Holder, it would have been someone else.) In this fight may be the seeds of opposition-party unity — at a time that a big batch of new spending is doing a fairly good job of giving the GOP a rallying point it’s comfortable with. At the very least, this will be the Obama team’s first glimpse of what its opponents are capable of, inside a committee that could shape the long-term contours of the incoming presidency. With the ghosts of Janet Reno and Alberto Gonzales in the hearing room, it’s going to be long, it’s going to be dirty, and it’s going to be ugly.