The recent run of opinion polls showing the Conservatives enjoying a small lead over Labour has understandably attracted a fair bit of comment. It’s rare for voting intentions to switch quite so quickly from one party to another, and seemingly off the back of a single conference speech.

But it’s worth remembering that the sort of Tory leads we have seen so far are nowhere near the kind David Cameron needs to be sure of ending up the largest party at the election, let alone forming the next government.

A good rule of thumb is that the Tories need at least a four-point lead over Labour to be sure of being the largest party, and at least a six-point lead to have a chance of getting a majority.

Even this wasn’t enough in 2010, when they had a seven-point lead over Labour and still fell short. But that was when the Liberal Democrats were polling above 20%. That’s not going to happen in 2015, so the targets are slightly lower.

Here’s an illustration of how our electoral system could mean the Tories win more votes than Labour on polling day, but still end up with fewer seats. I’ve used the New Statesman’s excellent election website May2015.com to come up with some scenarios based on Labour achieving a range of different vote shares.

In this first table, I’ve kept the Tories at 35% and the Lib Dems at 14%, but varied Labour’s share between 35% (the maximum I think they’ll be able to achieve) and 29% (the share it won in 2010):

You can see that it’s not until Labour is on 31% – four points behind the Tories – that it is no longer the largest party. And even with a six-point lead, the Tories are still only on 314 seats: 12 short of the 326 needed for an overall majority in parliament.

In this second table, I’ve moved the Tories up to 37% and kept the Lib Dems on 14%. Once again it’s the case that not until the party is four points ahead of Labour does it win enough seats to become the largest party. And I have to take Labour right down to 29% – a whole eight points behind – for the Tories finally to win enough seats to inch past the 326 target and form a majority government:

Of course, all these calculations are based on uniform swings and don’t take into account the likely impact of Ukip or the SNP on individual seats, nor the chances of the Lib Dems performing better in some places than others, and so on.

But hopefully they give a sense of the struggle the Tories face in opening up a sustained lead over Labour large enough to give them a fighting chance of, at the very least, forming a new coalition government.