“Can you imagine,” he said wistfully, “that moment when we stand on stage and the returning officer reads out the result … a 25-year-old local resident has unseated the prime minister for the first time in history.”It’s not as far-fetched as it sounds. What was traditionally a safe Conservative seat has in recent years shifted to marginal. In the last election in 2017, Johnson’s majority more than halved, to just over 5,000 votes.