Usually, I find it hard to empathise with Lib Dems, but I'm going to give it a go. Let's say you're a Lib Dem MP in a Labour wasteland with the Tories breathing down your neck. Your Scottish leader just did a conference speech attacking Cameron and setting out the five conditions Labour would need to meet for the Dems to coalesce with them after the next election.
I would be LIVID. After that speech, it is crystal clear that a vote for the Libs will be a vote to keep Labour in power. Vote Ming, get Gordon. The following 20 MPs are most at risk from Tories mopping up the anti-Labour vote in their area. I can't believe they will allow Ming to kill their careers. The choice for them is stark - mutiny or retirement.
Sandra Gidley - Romsey & Southampton North (Maj 100)
Lorely Burt - Solihull - 300
Tim Farron - Westmorland & Lonsdale 300
Jeremy Browne - Taunton 600
Tom Brake - Carshalton & Wallington - 1000
Chris Huhne - Eastleigh - 1000
Paul Keetch - Hereford & South Herefordshire - 1000
David Heath - Somerton & Frome - 1000
Martin Horwood - Cheltenham - 2000
Adrian Sanders - Torbay - 2000
Dan Rogerson - Cornwall North - 3000
Mike Hancock - Portsmouth South - 3000
Paul Burstow - Sutton & Cheam - 3000
Roger Williams - Brecon & Radnorshire - 4000
Susan Kramer - Richmond Park - 4000
John Pugh - Southport - 4000
Don Foster - Bath - 5000
Nicholas Harvey - Devon North - 5000
Annette Brook - Dorset Mid & Poole North - 5000
I should note that there are Lib/Con coalitions that have operated well. I wander what John Hemming Birmingham MP and Lib Dem Councillor thinks of this.
Update - The names were based on 2005 results. Factual edits have been made as Cheadle is no longer marginal. Of course, many of the above seats may also be impacted by boundary changes/MPs standing down etc.