What would be a good result for the various political parties in tonight’s local elections?

I wrote in greater length about what Labour and the Liberal Democrats should look for before the announcement of the snap election, but that the election is in five weeks not two years has changed the game as far as what a good result would be.

Why? Because the long-and-the-short of it is that voters tend to use local elections in years without a general election to punish the governing party, boosting the performance of the opposition parties. When local and general elections are held on the same day, people tend to vote for the top of the ticket and then the same way in the council elections on the same day.

The Conservatives’ 2015 performance is a handy example of this. David Cameron’s Tories gained 541 councillors that night. In contrast, in 2014, they lost 236, in 2013 they lost 335, and in 2012 they lost 405. In 2011, an usually good year for the governing party, they actually gained 86, an early warning sign that Ed Miliband was not on course to win, but one obscured because of the massive losses the Liberal Democrats sustained in 2011.

The pattern holds true for Labour governments, too. In 2010, Labour gained 417 councillors, having lost 291 and 331 in Gordon Brown’s first two council elections at the helm. In 2005, with an electoral map which, like this year’s was largely unfavourable to Labour, Tony Blair’s party only lost 114 councillors, in contrast to the losses of 464 councillors (2004), 831 councillors (2003) and 334 councillors (2002). This holds true all the way back to 1979, the earliest meaningful comparison point thanks to changes to local authorities’ sizes and electorates, where Labour (the governing party) gained council seats after years of losing them.

So if the election was being held in three years’ time, Labour would need to comfortably exceed the performance necessary to win a general election to be on course to defeat the Tories in 2020. To give you an idea of the challenge: two years into the last parliament, Labour’s projected national vote share after the local elections was 38 per cent. In 2015, Labour got 31 per cent. In 1985, Labour’s projected national vote share based on the local elections was 39 per cent, they got 30 per cent in 1987.

The falls in the performance of the Conservatives in opposition were smaller but still significant – they dropped four points from the local elections in 1999, 2003 and 2007 in the general elections of 2001, 2005 and 2010.

The picture is even more more stark when you consider the leads that the opposition parties enjoyed as far as the local elections in 1985, 2007 and 2012 go. In 2012, Labour enjoyed a national vote lead of seven points. They lost the 2015 election by five points. In 2007, the Conservatives had 13 point lead over Labour. They won the 2010 election by seven points. In 1985, Labour led by seven points – which turned into a deficit of 12 points by 1987.

So that gives you an idea of the level of cushion Labour need to get in a local election two years after the last election to feel optimistic about the next election.

But because of the coming general election, voters will behave differently – although the difficulty here is that the dataset is more limited. There are only three meaningful sets of results to compare with. In 1983 and 1987, local elections were held with the expectation but not the certainty of an election that year. In fact, Labour had gone so far as to prepare a slogan and a campaign poster for a May 1987 election. In 1992, local elections took place a month after the Conservatives’ unexpected fourth election victory.

In all three, voters behaved a lot more as if they were voting on the same day as the general election. The Conservatives won all three sets of local elections. But in 1983 and 1987, the governing party underperformed its general election performance, and the opposition still overperformed, but not by as great a margin.

All this by way of saying: to have a reasonable expectation of winning the election 8 June, Labour would need to win the local elections and win them well. It’s not certain- but it’s pretty damn likely that they will not get above their projected vote share from the local elections today on 8 June. It’s highly unlikely that the Tories will fall below theirs.