Another batch of opinion polls from marginal seats are due on Tuesday, once again courtesy of Lord Ashcroft.

This time he’s surveyed the state of play in Liberal Democrat-Labour battlegrounds. If he follows the same approach as before, we should get polls from Lib Dem seats where Labour are in a close second place, plus seats where the reverse is true and the Lib Dems are close behind Labour.

It’s long been speculated that the Lib Dems are likely to lose a fair few constituencies to Labour at the 2015 election. There’s an expectation that a sizeable chunk of voters who switched from Labour to the Lib Dems in 2010 will switch back next year – one backed up by trends in opinion polls as well as the performance of both parties in recent local elections.

Lord Ashcroft’s forthcoming polls will at the very least add to this speculation, besides giving us snapshots of how things currently stand in some of the key marginals.

Here are the Lib Dems’ seats most vulnerable to Labour:

Look at how large the Lib Dem majority is in Lynne Featherstone’s seat of Hornsey & Wood Green, the 10th seat on the list. The equivalent seat on the list of Lib Dem-Tory marginals has a majority of 2,981. In other words, the Lib Dems have fewer marginals vulnerable to Labour than to the Tories – and Labour has got to achieve a bigger swing to take all 10 on this list (around 5%) than the Tories need to take all 10 on their list (around 3%).

Saying that, the party did do better in London during May’s local elections than elsewhere in the country, especially against the Lib Dems. Hornsey & Wood Green is in the capital. I can foresee it falling to Labour while others on the list, such as Edinburgh West and Redcar, may remain Lib Dem.

By extension, I wonder whether Simon Hughes in Bermondsey & Old Southwark could be at risk from Labour, despite having a majority of 8,530.

For comparison, here are the Labour seats most vulnerable to the Liberal Democrats:

Note how the majorities are all below 2,000. Only five of the seats on the Lib Dems’ list are below 2,000. Labour will be defending an awful lot of marginal seats next year, never mind trying to gain seats from rivals.

That figure for Oldham East & Saddleworth is Labour’s majority at the 2010 election. In the by-election in 2011 Labour increased this to 3,558, but come the 2015 election any change will be measured against the 2010 result. (This goes for all the other seats contested in by-elections since the 2010 general election, including Corby which Labour gained from the Tories in a by-election in 2012).

One last comparison. A total of eight of those 10 Labour-Lib Dem marginals have majorities below 1,000. But on the list of the 10 Tory-Lib Dem marginals, only four have majorities below 1,000.

Labour will need to work harder, achieve bigger swings and pile up more votes than the Tories to win in their 20 key Lib Dem battlegrounds. But if they do, these will be the places where the foundations for the next coalition government (which is what I think it will be) are laid.