Ed Miliband benefits when leftwing voters desert Nick Clegg but he suffers when moderate positions on Europe and immigration are shunted to the margins.
The Lib Dem leader should seek a renewed mandate from his party.
Clegg is unwilling to fall on his sword and the party isn't prepared to force him out.
A year after almost winning the Eastleigh by-election, Ukip didn't gain any council seats there.
The party held onto Eastleigh and Sutton but lost Cambridge and Kingston.
Vince Cable says the Lib Dem leader took a "gamble" by campaigning on Europe, while Lynne Featherstone says her party has lost its "humanity".
Cameron fears the collapse of the coalition. Labour fears a more popular centre-left leader.
The party's pledge to build 300,000 homes a year is barely known.
Party president also says Lib Dems will not withdraw from the coalition.
It would enhance his bargaining power, while the Tories can remind him that they allowed a vote on AV.