Tuesday 4th May 2010 | 16:30

Poll centre projections: battleground scenarios

Rob Ford, co author of the PoliticsHome projection model, examines how different patterns of Conservative performance in the marginals would affect their projected seat shares

Our model assumes that the Conservatives will pick up an extra two points of swing in the key battleground marginals they need to win a majority – the 61 seats where a swing of between 3 and 7 points are required to win the seat from Labour*.  

However, in the weeks since the first leaders’ debate the polling has become much more volatile, including in the marginals. How would our projection change if the pattern of swing in the marginals were different? I examine this issue in two ways. First, I look at how the strength of Tory performance in the currently defined marginals battleground influences the seat shares projection. Then I look at what would happen if the Labour-Conservative battleground is expanded.

1. The returns to a strong marginals performance

Our current projection is for the Conservatives to win 293 seats, 33 short of the number needed for a Parliamentary majority. How would that number change if the Tories did better – or worse – in the key marginals battlegrounds?

Table 1 shows how the seat projection changes as we alter the Conservative marginals bonus. The conservatives gain about an extra five seats for every additional point of swing in their key battlegrounds.

However, this gain begins to tail off as the swing bonus rises above three points, because at this point most of the key battlegrounds already fall into Tory hands. Even with a five point swing bonus in their key battlegrounds, we would project the Conservatives to get under 300 seats based on their current poll shares. If the Tories turn out not to perform better in the battlegrounds than elsewhere we would project them winning 281 seats, ten less than our current forecast.

Battleground swing bonus	Con seats	Lab seats
-2	272	249
-1	276	244
0	281	240
1	286	235
2	291	230
3	295	225
4	298	224
5	299	223

 

2. Expanding the battleground

Many Conservatives have claimed that the events of the past few weeks have expanded the battleground of seats where the party can effectively compete with Labour. What would happen if the Tories outperformed across a wider battleground of seats?

Table 2 illustrates what our model would project in two broader battlegrounds. In the expanded battleground, we award the Conservatives extra swing in all Labour-Conservative marginals where Labour have a majority up to 17 points.

This brings in seats such as Jon Cruddas’ Dagenham & Rainham into the Tories’ cross-hairs. The second “super size” battleground takes in an even larger range of seats, covering those where the Conservatives are trying to overturn Labour majorities of up to 21 percent. This battleground takes in seats such as Jack Straw’s home seat of Blackburn and Ed Balls’ Yorkshire seat of Morley and Outwood.

The table reveals that, as we would expect, a broader battlefield will yield more Conservative gains. However, the returns from broadening the battlefield are fairly modest unless the Conservatives achieve a large swing bonus in these seats. This is because on current polling, the Conservatives are quite a long way short in many of the expanded battlefield seats and so they need to outperform by a long way in order to stand a chance of winning.

If the Tories outperform by just one point in the battlefields, they win 286 seats using the standard battleground, and a further 6 seats using the super size battleground. With a two point battleground bonus, Conservative seats stand at 291 with the standard battleground and ten higher at 301 with the supersize battleground. If the Conservatives rout Labour in the battleground seats, then a broader battleground delivers a better return – with a five point swing bonus in battleground seats and a “super-sized” battleground, we project the Conservatives to be close to a majority on current polling.

Recent marginals polling data we have suggests the bonus the Tories are winning in the battlegrounds is much more modest than this. At present, we do not feel there is sufficient consistent data to alter our definition of the battleground where we award a marginal bonus, or the size of that bonus, in our central prediction.

However, that prediction is only one possible estimate and we hope these additional tables will help readers understand the possible impact of the battleground. A bigger battleground, or a stronger battleground performance, will not be enough to win the Conservatives a workable Parliamentary majority. They also need to outperform their current polling shares.

Table 2: The impact of swing bonuses in three different battlegrounds

Battleground swing bonus	Con seats, standard battleground	Con seats, expanded battleground	Con seats, super size battleground
0	281	281	281
1	286	290	292
2	291	298	301
3	295	303	309
4	298	308	318
5	299	312	325

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*We do not apply the bonus below the 3 point level because the Conservatives are very likely to win the majority of these seats, and so they are not pivotal in deciding the outcome. Applying an adjustment here has little effect anyway, as our model projects them as very likely Conservative gains.