Rob Yeung's Answer:
I can understand that you feel your job may be under threat, but the way to get through this situation is to look at it from your boss's perspective.
Your boss is likely to want what is best for the team and the rest of the organisation. So it would be irresponsible of your boss not to listen to ideas from other people in the team as to how to cover your work while you are away. So my first point is not to take it personally.
Once you understand that your boss is probably looking to do what is best for the team, you can decide how to tackle the situation. I recommend that you ask your boss for a meeting to decide how best to manage your temporary absence. Ask your boss to invite any people who might have suggestions as to how to cover your leave.
By inviting the office politician and would-be empire-builder, you should get to understand his or her agenda. At that meeting, ask lots of questions. Even if you disagree wholeheartedly with what he is suggesting, make neutral statements such as "that's an interesting point of view" or "that's something to think about".
If you try to argue with his point of view, you risk driving him underground only for him to resurface with his views after you are gone. Better to get his agenda and points of view out in the open while you are still around to do something about them.
Once you have heard all of the concerns, say that you will draw up a plan of action that you can all buy into. Take the office politician's points and think how you can structure the team or your work in a way that both meets your needs and furthers the needs of the team and wider organisation. Create a plan of action that your boss and the conniving colleague can buy in to.
But you must remember that what you propose must meet the needs of the organisation and not just protect your own interests. Otherwise you will appear as guilty of protecting your own empire as your colleague is of expanding his.