The Liberal problem has always been that they have increased their Westminster representation at the expense of a weakened Tory party for the past decade, but failed to commit their voters to a new, distinctive Liberal position. They have grown rich on the soft pickings of a declining Tory party, but it was always going to be the case that once the Tories revived, their newly acquired seats would disappear back into the natural middle-class fold of the Tory Party. The Liberals look increasingly like the British political equivalent to the Eastern Europe nations between the wars - created when their powerful neighbours were weak, it was only a matter of time before a resurgent neighbour gobbled them back up.
The Liberals have functioned as a protest party for disillusioned, moderate Tories, and have in addition inherited Tory seats with apparently healthy majorities on the strength of low turnouts. A resurgent, moderate Tory party under Cameron means not only the potential return of some of the protest vote, but also the return to voting by the hitherto absent Tories. There is no easy way for Clegg to combat this. In fact, the real answer for the Liberals was probably to take advantage of New Labour's move into the centre ground under Tony Blair and Gordon Brown by seeking to colonise the left of the political spectrum more effectively. There are a huge number of disillusioned, disenfranchised voters in ghetto-ised urban areas which a genuinely radical Lib Dem party could have reached, creating a much stronger, more lasting majority for itself. Successive Liberal leaders simply failed to grasp this opportunity, and in choosing to follow the other two parties into the centre-right, they will reap the rewards of their callow opportunism at the next election.