Analysts said the North Shropshire vote concentrated minds on Johnson’s ability — or not — to win elections. “His whole premiership is based on, ‘he’s good at winning elections,’” said Rob Ford, a professor of politics at the University of Manchester. “If it becomes a settled view that, far from being an electoral Gandalf, he is an electoral Voldemort, he’s not long for Number 10,” he added, referring to the prime minister’s address at 10 Downing St. in London.