I have mainly concentrated on the Labour/Tory battleground. I make no apology for this: this is where the election will be decided and this is where most of the constituency betting opportunities are. But for completeness, I ought to put up a brief post on the Lib Dem/Labour marginals.
I have already looked at the likely behaviour of 2010 Lib Dems in these seats. I came to the conclusion that those voters who have defected to Labour have made their minds up and will not be returning. This leaves the question whether the Lib Dems can encourage more Conservative supporters to lend them their votes.
In favour of this notion, Tories who had previously thought that the Lib Dems were surrogate socialists may have changed their minds following five years of coalition. Some prominent Lib Dems have enthusiastically made the case for austerity - Danny Alexander for one. (Others, however, have been Hamletesque in their public anguishing over getting into bed with the evil Tories.)
Historically, the Lib Dems have not had much success at this (in stark contrast to their ability to harvest Labour tactical votes when they are competing with the Conservatives). I attach a list of the Lib Dem seats where Labour are in contention:
The average Conservative vote share in these seats is nearly 19%. Admittedly, four of these seats have Conservatives in second place at present, but even without those four, the average is still 17%. The Conservative vote in these seats has remained resilient.
To date, the polls have betrayed no hint that such Conservative voters are amenable to the idea of tactical voting, but that is not necessarily that informative, since these voters have not thought about the idea before and they may not have processed the concept as yet this time either. Will they do so this time around?
This is one of the unknowns of next election. My default expectation is that we will not in general see the Lib Dems tap into this potential voting resource. Too many Lib Dem MPs have been publicly too lukewarm about the coalition to inspire any kind of affinity from Conservative supporters. Before 2010, many Labour supporters saw the Lib Dems as a kid brother to their own party. The Lib Dems in general have done too little to inspire a similar light affection in the minds of Conservative supporters.
As an added disadvantage, with the heavy swings against the Lib Dems, in many seats Conservative supporters will reasonably conclude that the Conservatives stand at least as good a chance as the Lib Dems of getting elected. Why vote for ginger beer rather than champagne when champagne is also on the menu?
Some MPs may be able to appeal credibly for tactical Tory support. Anecdotally, Danny Alexander and Nick Clegg have both been seen as positive contributors to the coalition by Conservatives. Nick Clegg hardly needs the votes, but Danny Alexander does. But his is a constituency where the Conservative vote has already fallen to a relatively low 13.3%. I am doubtful whether he can get that all that much lower.
In summary, while this in theory looks like a tempting source of votes for Lib Dem incumbents, in practice I think they are going to find these voters tough going to convert. I'm very bearish about Lib Dem chances in such seats in general and so my betting approach is generally to be very cautious indeed about backing Lib Dem chances in such seats, and to consider strongly backing Labour challengers in these seats even where large swings are required. Having lost the casual affection of many left of centre voters, they have failed to gain in compensation the casual affection of right of centre voters. There is only so far that a personal vote can take you. If the Lib Dems are holding half of these seats this time next year, they will have done very well indeed from the current starting point.