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I think the Lib Dem vote collapsed this time because people associated voting LibDem with a real possibility of helping the Tories get back in (with another Con/Lib coalition) and that despite the moderation they did manage to do, many people think they didn't dig their heels in enough. A single broken pledge on University fees wasn't alone worth losing 2/3 of your voters.
Likewise Labour's vote collapsed (in many places) as the Tories had fear-campaigned that a vote for Labour was a vote for a Lab/SNP coalition.
Since the election results have been out both Labour and LibDem have seen a surge in new memberships.
I don't think it was necessarily the coalition aspects that did for the LibDems, more the lying about tuition fees. I actually have some (limited) sympathy for Clegg's contention that they moderated some of the worst bits of the Tory government - human rights and snoopers charters spring to mind. Although they did wave some other bits through...