There is a very simple reason why Lord Adonis has called on Liberal Democrat supporters to vote tactically for Labour. Lots of them have done so at recent elections, and if they stop doing so now, such hopes Labour has of coming ahead in seats on 6 May are likely to be dashed.
Third-placed Liberal Democrat supporters have switched significantly to Labour in order to defeat the local Conservative at recent elections. The pattern first became noticeable in 1992, when on average around 1 per cent of the vote seems to have switched away from Lib Dem to Labour in key Conservative-held marginals, thereby costing the Tories half a dozen seats.
The phenomenon became more marked in 1997, by which time Paddy Ashdown had signalled he preferred Labour to the Conservatives. As much as 3 per cent of the vote often transferred from the Lib Dems to Labour in seats the Tories were defending against a Labour challenger. As many as 20 newly elected Labour MPs owed their victory to such tactical support.
Meanwhile in 2001 there was yet a further 1 per cent or so switch of votes from the Lib Dems to Labour in some of the most marginal Lab/Con contests. More importantly, neither in that election nor in 2005 was there any evidence of past tactical support for Labour unwinding back towards the Lib Dems – even though by 2005 Labour was far less popular and relations between the two leaderships had cooled.
So Labour has a substantial legacy of past tactical support it has to retain in many of its key marginals. If it were to unwind, the swing to the Tories in these seats would be above the national average, irrespective of whatever impact the controversial funds supplied to Tory challengers by Lord Ashcroft eventually have.
There are at least three reasons why there might be such a "tactical unwind". The first is rather technical, but no less important for that. Across much of England and Wales the election is being fought on new constituency boundaries. This means some voters who were previously in a relatively safe seat now find themselves in a more marginal one. Where this has happened, Labour will have to persuade some Lib Dem supporters to switch to them for the first time.
The second is more obvious. The Conservatives are more popular than they were in 2005. This change of mood is reflected in the attitudes of Lib Dem voters. At the last election, only around a quarter preferred the Tories as their second choice while over half favoured Labour. Now when Lib Dem voters are asked if they would prefer a Labour or a Conservative government, only slightly more say Labour than Conservative.
So, while Lib Dem supporters are still more favourably disposed to Labour than voters are in general, they are more balanced in their views than five years ago. There thus seems less reason to anticipate that when Lib Dem supporters do decide to vote tactically they will necessarily do so in favour of Labour.
Finally, unlike his predecessors, Nick Clegg has a clear incentive to attack Labour rather than focus his fire on the Tories. The 2005 election saw the emergence of a whole new set of Labour/Lib Dem marginals. Of the Lib Dems' top 20 targets, no less than nine are held by Labour. And if Lib Dem supporters hear Mr Clegg attacking Labour, they might wonder whether they are still worth a tactical vote after all.
John Curtice is Professor of Politics at Strathclyde University. He is analysing the polls for The IndependentReuse content