Nate Silver sketches out the Lib Dem dilemma. If Labour bleeds enough support for the Lib Dems to get more seats than Labour, the Tories will probably have gained enough Labour votes for a majority:
Obviously the [Lib Dems] want as much of the vote as possible. But their realistic upside is probably a hung parliament in which Labour does embarrassingly poorly and they become the de facto second party. If the model is right, however (yes, I'm going to keep caveating that), and if Cameron gets another point or two out of the debate, it may be hard for them to overtake Labour in the seat count whilst simultaneously denying Conservatives a majority.