Clegg bet big on a referendum to replace First Past The Post with Alternative Vote. He believed that this would greatly increase the power of third parties, especially his third party.

It was rejected by a 2:1 margin.

https://en.wikipedia.org/wiki/Parliamentary_Voting_System_an...

It's possible that it could have been a huge win, which would have enabled all of the rest of his (and the LibDem's) agenda. Instead, it basically destroyed the party.

His successor tried to resuscitate it, when Labour failed to be an anti-Brexit party. That was a huge opportunity. But they were so far behind after Clegg's debacle that all it really did was to turn some Labour seats to Tory.