The forthcoming Eastleigh byelection triggered by the resignation of Chris Huhne pits the Liberal Democrats and the Conservatives against each other in a proper gloves-off dust-up for the first time since the two parties formed a coalition government nearly three years ago.

In this respect it promises to be an intriguing early rehearsal of one of the key issues of the 2015 general election: to what extent will the Liberal Democrat vote collapse, how many seats will the party lose as a result, and who will reap the benefit?

The more the Lib Dems haemorrhage seats, the more David Cameron can hope for that fabled parliamentary majority that on the face of it looks so unlikely today.

To get a sense of what might happen in 2015, I’ve gone through the election results in the 57 seats that the Liberal Democrats won in 2010 and ranked those seats according to the majority achieved (the lead in share of the vote over the second placed candidate), starting with the narrowest victory.

The Lib Dems’ most vulnerable seat is Solihull, where Lorely Burt won with a mere 0.3 percentage point lead over the Conservatives; at the bottom of the list is rock-solid Orkney and Shetland, where Lib Dem chief whip Alistair Carmichael enjoys a lead of 51.3 points. Eastleigh ranks as the 20th most vulnerable seat.

For each seat I’ve noted the margin of victory and the party that came in second place.

What is striking is that the Conservatives stand to gain much more than Labour from a Lib Dem meltdown. Tory candidates came second in 38 Lib Dem seats, to Labour’s 17 (the SNP and Plaid Cymru each had one runner up too). More significantly, the Conservatives are leading the chase in the more vulnerable seats: in seven of the top 10 and in 22 of the top 30.

Let’s assume a uniform swing of 5 percentage points away from the Lib Dems across all their seats was matched by a 5 percentage point gain by the second placed party in each of their seats – and for the sake of argument that all the other parties were to win the same share of the vote as before.

In such a scenario, the Lib Dems would lose 27 seats, leaving them with just 30 MPs at Westminster. The Tories would pick up 19 of these. Add those to the 306 MPs they won last time – assuming, of course, they were to win all of those again – and they would reach 325 MPs, exactly half the House of Commons.

Push the swing to 6 percentage points, and the Tories would pick up another three Lib Dem seats, taking them into majority territory.

This kind of drop in Lib Dem support is hardly the stuff of fantasy: remember they won 23 per cent of the vote in 2010 and no recent poll has put them anywhere near that. UKPollingReport currently has them on an average rating of 10 per cent.

Caveats abound: will the Lib Dems rally support locally even if their nationwide share is hammered? Will there be a surge in Tory support at all, let alone one that matches the Lib Dems’ losses? Might Labour come through to win from third place in some of the three-way marginals?

And how many seats where the Tories face a close fight with Labour will they hang on to? We can be pretty confident that Labour’s share of the vote is going to rise from last time’s dismal 29 per cent. And then there’s the Ukip factor.

For all these vagaries, what happens to the seats the Lib Dems lose – how many they lose and who picks them up – is going to be crucial come election night.

Cameron has a Herculean task in getting a majority – the system is heavily skewed in Labour’s favour, especially now that the Tories’ planned boundary reforms have gone down in flames. No incumbent Tory government has managed to increase its share of the vote at a general election since 1955.

His wildcard is Scotland, because a yes vote in 2014 leading to independence in 2016 (on the SNP’s timetable) makes it hard to imagine Scotland returning MPs to Westminster in 2015. Let’s park that for now, though.

Cameron’s hopes of a majority rest in no small part on a Lib Dem wipeout. And to get a foretaste of this defining contest we have just two weeks to wait… for Eastleigh to give up its secrets.

Andy JS

My Conservative Target List for the next general election:

https://docs.google.com/spreadsheet/ccc?key=0At91c3wX1Wu5dEk1TlVqMHhNUXFBWlhSNU1hd0FYSHc#gid=0

Andy JS

This is the Labour equivalent:

https://docs.google.com/spreadsheet/ccc?key=0At91c3wX1Wu5dDRiT1FSRTF2bjVYRThSTnRaNzFXMlE#gid=0

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