[James]

First and foremost a leader of any major party has to have the backing of its own MPs, so the leader can form policies, and put them to the public under the party name at elections.

Ed Milliband has left the Labour party with a pretty flawed system for voting a new leader which doesn't give MPs much of a say. They could end up with a leader who doesn't represent the views of his own MPs. Which could lead to a split with a new SDP being formed.