Lord Ashcroft has published his survey of Lib Dem-Labour battlegrounds.

It proved to be somewhat smaller in scope than his previous study of Lib Dem-Tory contests. He polled only four marginals, which happened to be the four needing the smallest swing from Lib Dem to Labour. Unsurprisingly, the results suggest Labour is currently on course to win all of them rather effortlessly.

Labour needs only a 2.1% swing to capture this quartet. Lord Ashcroft’s polls found the following swings:

Norwich South: 10.5%
Bradford East: 11.5%
Brent Central: 19%
Manchester Withington: 19%

The average swing is 15%, which when extrapolated across the full list of Lib Dem-Labour marginals, implies Labour could win up to 17 seats from the Liberal Democrats.

All of the seats on my “10 most vulnerable” list would change hands, along with half a dozen or so more.

A few things to note about this prediction.

Firstly, it won’t happen. I mooted the other day the possibility of seats like Hornsey & Wood Green and Bermondsey & Old Southwark (both in London) falling to Labour, while others with smaller majorities – like Edinbugh West – staying Lib Dem.

It would’ve been useful if Lord Ashcroft had polled a few of these seats, rather than just the uber-marginals. But his average swing of 15% is so high as to invite caution, and he himself concedes that swings are very far from uniform where the Lib Dems are concerned. Remember his survey of Lib Dem-Tory marginals found certain seats bucked the trend, such as Sutton & Cheam and Cheadle.

Second, Ukip doesn’t seem to be a threat to Labour in these seats. Here’s where Ukip came in the four polls, compared with Labour’s lead over the Lib Dems:

Norwich South: 15% (Lab-Lib Dem lead 21%)
Bradford East: 15% (Lab-Lib Dem lead 22%)
Brent Central: 7% (Lab-Lib Dem lead 35%)
Manchester Withington 4% (Lab-Lib Dem lead 34%)

The very low ratings for the London and Manchester seats repeat the pattern of May’s local elections, which saw Ukip do particularly poorly in large metropolitan areas.

If there is a “third party” that could upset the trend or the swing, it is the Greens. Lord Ashcroft’s poll of Norwich South found the Lib Dems had gone from first to fifth place, behind Labour, the Greens, the Conservatives and Ukip in that order. Overall he found that for every two Lib Dem voters switching to Labour, one had also gone to the Greens.

Here’s the Norwich South poll in full:
Labour 33% (+4)
Green 20% (+5)
Conservative 18% (-5)
Ukip 15% (+13)
Lib Dem 12% (-17)
Others 1% (-1)

Lord Ashcroft also did a poll in the seat of Green MP Caroline Lucas, Brighton Pavilion, which showed Labour one point ahead of the Greens. I think Lucas will hold on here, but I’ll return to look at this contest more closely another time.

Might it be the case that the Lib Dem marginals are wiped out in a rather lopsided fashion, with more falling to the Tories than Labour? The number of Lib Dem majorities below 1,000 on the list of Tory target seats is greater than that on Labour’s list. Thanks to the larger Ashcroft survey of Lib Dem-Tory battlegrounds we have a slightly better idea of the scale of constituencies that could fall to the Conservatives. And the Green vote could deny Labour some of its Lib Dem long shots.

Either way it’s bad news for the Lib Dems, who – if both surveys come to pass in full – stand to lose half of their current MPs.

Lord Ashcroft has said he will repeat the polls in both sets of battlegrounds later in the year.