As Theresa May learned to her cost two years ago, it’s not just, or even mainly, policies that decide elections, but party leaders and their character. In 2017, May’s ratings crashed during the campaign, while millions of voters discovered that Jeremy Corbyn was not as dreadful as much of the media had painted him.
Political scientists have a word for this election-changing phenomenon: “valence”. In their prime, Margaret Thatcher and Tony Blair won huge victories because they were clear valence-victors. They were seen to be tougher, more competent, more principled and more in touch with voters than their rivals.
These qualities matter, because only a minority of voters pay close attention to the twists and turns of detailed policy developments – and they seldom decide elections because they generally have firm party loyalties. The voters who matter – the ones who are apt to change their minds during election campaigns – tend to be not so much “policy” voters as “valence” voters. They form a broad view as to which parties, and above all their leaders, have the competence and character to provide Britain with effective leadership.
With an autumn election in the offing, Deltapoll’s survey shows that Boris Johnson is winning his valence war with Corbyn – but more because the Labour leader’s character ratings are weak than because the Prime Minister’s ratings are strong.

Source: Deltapoll; sample 1,089; fieldwork August 8-12
In fact Johnson scores well on the purely political virtues – a willingness to take tough decisions, strength of character in a crisis, and having “what it takes to get the job done” – but far less well on more personal characteristics. Fewer than one in three thinks he can be trusted to tell the truth or “understands the lives of people like me”. Just one in four “would be happy for him to look after my young children”.
The prospect of the Prime Minister baby-sitting for a floating voter in a marginal seat is not particularly high: but responses to this kind of question can reveal voters suspicions of character defects that might creep in time into voting choices.
If the news for Johnson is mixed, it is almost uniformly bad for Corbyn. He has terrible ratings on strength of character, getting the job done and perceived truthfulness. Voters reject him as an emergency baby-sitter as decisively as they reject the Prime Minister.
Corbyn is in positive territory on just two qualities: sticking to his principles and believing in fairness and equality. However, instead of striding well ahead of Johnson, as he might have expected, he enjoys only a narrow lead on fairness, and actually lags the Prime Minister on being principled.
So, in these early days of Johnson’s government, the Prime Minister is ahead on points. However, these figures are not set in stone. Just as in 2017, a fiercely fought election campaign will expose both leaders to intense scrutiny, and could cause many voters to change their views.
Will Johnson strengthen his reputation further – or will his current perceived character strengths start to fade? Can Corbyn, unlike a souffle, rise twice?
Plainly, an early election will be dominated by Brexit. But the fate of many closely fought seats, and thus of the overall election result, will depend on such things as Labour’s ability to retain its support in key marginals – and whether the Tories in these seats can prevent defections to Nigel Farage’s Brexit Party. And these factors in turn will depend in large measure on which of the two leaders wins the valence war between now and polling day.