Every political party has what's referred to as a "core vote". This is the percentage of the public that can always be counted to vote for the party no matter what - e.g. the die hard voters who'd only stop voting for the party if they made their official policy the introduction of anthrax into free school meals, and maybe not even then.
The core vote isn't constant, it varies up and down. The reason the core vote matters is because, at the moment, the share of the vote required to have a working majority in parliament is about 40% - give or take a few percentage points. So obviously, the higher the number of people you can be guaranteed to vote for you, the less extra people you need to convince in order to win and the easier your job is.
Now, the core vote is very difficult to calculate but the best clue can be given by the number of people who say that they identify with a party in surveys. One of the most reliable surveys for this is the British Election Study that is carried out following every general election.
Here we can see that the number identifying with (and therefore as the core vote of) the two main parties has fluctuated slightly since 2005 but remains at approximately 25% for the Conservatives and at approximately 31% for Labour. The Lib Dems, by contrast have had a rock steady 11% identifiers in both 2005 and 2010. This is also born up by the fact that, during the nadir of Lib Dem unpopularity during the tuition fees debacle, the Lib Dems never dropped below 9% in the polls. 9% is obviously less than 11% but it should be born in mind that polls are only ever accurate to 2 or 3 percentage points and that the polling average for the Lib Dems never dropped below 10% and is currently at 11%.
So what does this tell us?
Well, it tells us that, as a party, we have a dependable core vote. The problem is that it is a third of the size of Labour's and a less than half the size of the Conservatives. The highest we ever reached in the polls during the election campaign was 30%. This shows that the potential size of our vote could be very high. But it also means that, if at the moment, we want to win elections, we need to convince 40% of people to vote for us in addition to our core vote. Labour and the Conservatives need only convince 5% and 10% respectively.
As such, it's obvious that the only way we'll ever be able to get beyond third party status is to grow our core vote. The 30% high in the polls shows us the highs to which we could potentially grow our core vote, now all we need is a way to do it.
That topic, incidentally, is the message of a two-part piece of mine that will be appearing on Liberal Democrat Voice starting next Monday. Do look out for it and I'll cross-post a copy on here for my regular readers.