At every general election we see the previous one as the benchmark against which to measure what happens this time. The problem with the 2017 election is that it is steeped in its own mythology, and this acts as a barrier to our understanding of the events unfolding as we approach polling day on 12 December.

The two previous elections witnessed a spectacular growth in voting for parties other than Conservative, Labour and Lib Dem. In 2010, other parties gained 10 per cent of the vote, which was itself a record; but in 2015 this figure rocketed to to an astonishing 23 per cent. Only two years later there was a seismic shift back again to the main parties, and this is where the myths were born.

In reality, 2017 was the losers’ election: the Conservatives lost their majority; Labour lost their third general election in a row in spite of the most disastrous Conservative election campaign imaginable; the Lib Dems received a derisory 7.6 per cent share of the vote (even lower than in 2015); the SNP lost a third of its vote and 21 seats; Plaid Cymru registered its lowest vote share in Wales in 20 years; and Ukip simply collapsed.

And yet for many, the 2017 result subsequently became a triumph for Labour – one, we are told, that will be repeated in 2019, but with a victorious outcome this time. How did this reshaping of history happen? The answer lies in the flooding back of voters to the Conservatives and Labour in 2017; but the myth would be sustainable only if that traffic was one-way (namely to Labour), and if you are allowed to cherry-pick your statistics.

Labour’s performance in 2017 was a remarkable triumph against expectations, in which they overcame the gloomiest of predicted fates. But escaping the noose is not the same as winning the battle of Waterloo. Labour’s share of the vote rose by 9.8 per cent and the party was able to claim that this put it just short of what it achieved in the 2001 election. But in 2001, Labour won a majority of 167 seats, whereas in 2017 they had no majority at all; indeed, they were 64 seats short of any majority. It does not require a degree in mathematics to spot the difference in outcome between these two elections. It is explained by the simple fact that is always missing from the mythology of 2017: the Conservative vote also increased substantially.

You would be forgiven for not recalling two records that Theresa May achieved in 2017. Firstly, she came within one percentage point of matching the share of the vote Margaret Thatcher achieved in her crushing 1983 election victory. Secondly, May secured the second biggest increase in the Conservative vote from one election to another since 1931. Yet she held no champagne celebration when these figures became known. Perhaps that is because she understood the difference between a vote share in 1983 that delivered a 140-seat majority and a similar one in 2017 that delivered defeat.

Top 10 marginal seats to watch at the general election Show all 10 1 /10 Top 10 marginal seats to watch at the general election Top 10 marginal seats to watch at the general election North East Fife Currently held by the SNP with a majority of 2 Getty Top 10 marginal seats to watch at the general election Kensington Currently held by Labour with a majority of 20 Getty Top 10 marginal seats to watch at the general election Perth and North Perthshire Currently held by the SNP with a majority of 21 Getty Top 10 marginal seats to watch at the general election Dudley North Currently held by independent, formerly Labour, MP Ian Austin with a majority of 22 LivingInMediocrity Top 10 marginal seats to watch at the general election Newcastle-under-Lyme Currently held by Labour with a majority of 30 Derek Harper Top 10 marginal seats to watch at the general election Southampton Itchen Currently held by the Conservatives with a majority of 31 Rob Candish Top 10 marginal seats to watch at the general election Richmond Park Currently held by the Conservatives with a majority of 45 Robin Webster Top 10 marginal seats to watch at the general election Crewe and Nantwich Currently held by Labour with a majority of 48 Jaggery Top 10 marginal seats to watch at the general election Glasgow South West Currently held by the SNP with a majority of 60 Alec MacKinnon Top 10 marginal seats to watch at the general election Glasgow East Currently held by the SNP with a majority of 75 Christine Johnstone

If we subtract the Conservative increased share of vote in 2017 from Labour’s increased share, we arrive at a 2 per cent swing to Labour – a rather pedestrian performance in postwar general election history and nowhere near the extraordinary 10.2 per cent swing to Labour in 1997, or Thatcher’s 5.3 per cent swing in 1979, or even David Cameron’s swing of 5.1 per cent in 2010. And nestling within that 2 per cent swing to Labour is the fact that by far the biggest swing to the party was in London (6.3 per cent) and the next biggest pro-Labour swings were in the southeast and southwest. These swings diminished the further one moved away from London, until in the northeast there was a slight swing from Labour to the Conservatives.

In past decades Labour had been haunted by its poor electoral performance in southern England; at this election it is troubled by how it will perform in its historic heartlands of the midlands and the north of England. And it should be: in 2017 Ipsos Mori recorded the biggest swing of middle-class voters from Conservative to Labour since 1979; and the biggest swing from Labour to the Conservatives among white working-class voters over the same period.

All of this has been swept under the carpet since 2017 as we have been encouraged to marvel at Labour’s mythological “victory in defeat”. But it just won’t do: Labour’s electoral performance since 2017, in European elections, Westminster by-elections, council by-elections and council elections, has been truly lamentable for an opposition party facing an inept government in disarray. Unless Labour understands why it was defeated in 2017 and acts accordingly, it is doomed to repeat the experience.

Myths are much more enticing than hard truths; but elections tend to be won by realists, not dreamers.