The importance of individual events can often be overplayed in politics. The key trends of voting behaviour are guided by slow-moving occurrences such as demographic shifts, changes of leaders, economic growth and time spent in office by the governing party. But referendums can move quicker than electoral politics, and the attention merited to Boris’s decision to come out and bat for leaving the EU – be that by forex traders or newspaper columnists - is understandable.
Why? Because of his appeal to the core group of swing voters who will decide the outcome of the referendum: moderate Conservatives.
To see how this is the case, let us start by looking at the building blocs of each side’s support. On the one hand, there are 3.8 million people who voted UKIP at the General Election last year, the vast majority of whom will vote to leave the EU. On the other, there are almost exactly the same number of Liberal Democrat and SNP voters, who are much more likely to vote to remain.
The Kippers are slightly more outty than Lib Dem and SNP voters are inny (there is only a very fine sprinkling of UKIP voters who want to remain in the EU, compared to between a fifth and a third of Lib Dems who want to leave). This gives a slight advantage to the Leave side, but is more than made up if you add a million or so In-leaning Green voters.
These two similarly sized blocs become two slightly larger similarly sized blocs when you include Conservative voters, who are currently split evenly (45% remain, 48% leave). The difference comes when you add Labour voters: around a quarter go to the Leave side, but the rest go to Remain. With the larger chunk of Labour voters going to the remain side, it leaves you with a noticeably bigger bloc of potential Remain supporters than Leave supporters, reflecting the relatively large lead for Remain seen in phone polls over the past few months.
With these blocs in place as they are, a victory for Leave will require sufficient numbers of people currently saying that they will vote for Remain to switch over to voting for Leave. There is only group which might realistically do this in large enough numbers to affect the result and that is Conservative voters.
With Corbynistas and Labour centrists unified behind Remain, it is not apparent what figure could emerge to persuade large chunks of Labour voters to switch in favour of leaving. The Lib Dem and SNP leaderships are overwhelmingly supportive of remaining in the EU, while the Northern Irish would have to vote in favour of leaving the EU near unanimously in order to tip the national result.
That leaves Conservative voters. They represent the largest bloc and are the most likely to be undecided. Half (52%) of Conservative voters say they may change their mind - more than enough to change the result. Of course many will not switch, but that only a third of Labour voters say they may change their mind reflects the higher levels of uncertainty among Conservatives.
This is true to past behaviour as well. The current split among 2015 Labour voters of 70% to 26% against Brexit is broadly consistent with how they broke back in September before the current bout of referendum fever took hold (70% to 23%). 2015 Conservative voters on the other hand, have moved in the past six months from 55% to 37% in favour of Remain, to splitting down the middle now (45% to 48%). These changes in opinion from some Conservative voters represent the main reason behind the wider narrowing of the race seen in the polls since the New Year. If this trend continues (and that is a relatively big if), it would see Conservative voters deliver victory for the Leave camp.
Not every Conservative voter is realistically persuadable – there is of course a significant proportion (16%) who are set on leaving the EU who can be excluded from the “swing vote” category, along those who are set on voting to remain (24%). That leaves just less than seven million “moderate” Conservative voters, who will determine the outcome of the referendum. And it is why Boris Johnson is so important.
He is liked by moderate Conservatives; jovial and far from the “nasty”, grey-haired male caricature of Eurosceptic Tory backbenchers. Just a third of Conservative voters (36%) are favourable towards Iain Duncan Smith, and just 24% favourable towards Michael Gove, so the Vote Leave campaign would have had a hard time persuading Tory voters had it been supported primarily by the Secretaries of State for Work and Pensions and Justice.
Two thirds of Conservative voters have a favourable opinion of the Mayor of London on the other hand (68%). This is vital as it means, thanks to his broader appeal than other Eurosceptic figures, Johnson can carry a message and moderate Conservative voters will consider it rather than dismiss it out of hand. With a hearing, comes the chance to change opinions. His affableness may also soften perceived threat of leaving the EU for some less committed remain supporters, reducing the incentive for them to go out and vote.
Not that Johnson will have an easy ride, as he found in the House of Commons yesterday. There is of course one towering figure respected more than any other by moderate Conservatives, whose endorsement for remaining in the EU is more important than any other: David Cameron. The Prime Minister has a favourability rating of 88% among Conservative voters (in comparison to other leaders, both Corbyn and Miliband have been rated favourably by only half of Labour voters). He may not be liked in the way Boris sometimes is, but Cameron commands credibility with moderate Conservatives. He is the politician Conservative voters are most likely to say will influence their vote on the referendum and prior to the General Election, he was far more trusted by voters to manage the economy than either Johnson or George Osborne. He has proved adept in the past at being a successful salesman with this group come crunch time too.
Remain still has the upper hand therefore, but with moderate Tories being all still to play for, the Mayor of London’s entry to the fray has changed what would have been a very one-sided affair into something which could become more of a scrap.