Lib Dem strategists are fond of telling reporters their assumptions about a Lib Dem collapse at the 2015 election are wrong. The theory goes that there are many more LD-Tory marginals than there are LD-Labour ones, and in those seats, Tory voters will be happy to back a Lib Dem to keep out Labour.
Lib Dem HQ hopes that by going into coalition and signing up to the stringent departmental spending cuts they will combine their traditional strength in local campaigning with a new level of trust to be a party of national government.
The argument is questionable: will Tory voters really back then Lib Dems, especially as the two parties go into the election playing up their differences to appeal to their core voters? And won’t lots of left-wing voters cross over to Labour, even if they know it means letting in the Tories, since many view the Lib Dem as just as bad as their Tory counterparts?
Whatever happens in those seats, there is another intriguing connotation of the Lib Dem vote collapsing, as noticed today by Mark Gettleson, a Lib Dem councillor and psephologist, writing for ConservativeHome.
In places where the party is third, they have generally not done a lot of local campaigning. This means those voters who do vote Lib Dem tend to have done so for national reasons. They tend, says Gettleson, to be former Labour voters who have switched over as a protest vote, against the Iraq war, tuition fees or civil liberties.
Those voters are now likely to return to Labour in their droves, as a vote for the Lib Dems can no longer be thought of as a left-wing protest.
What does this mean for the Tories? Gettleson identifies 33 Tory-Labour marginals where the Tory majority is smaller than the number of voters who have moved from Labour to the Lib Dems since 1997. If they switch back, as Gettleson expects, it will deal a heavy blow to Tory attempts to become the biggest party, let alone form a majority.