Back in 2015, a new M.P. from Streatham was among those running to replace Ed Miliband as leader of the Labour Party in Great Britain. This was Chuka Umunna, who suddenly pulled out, explaining that he didn’t want to put his wife through the ordeal of the campaign and his possible win. He worried, he said, about her loss of privacy. He was also 36 at the time, and may have felt unready; he knew he had plenty of time to prepare for a run. As it happens, the Labour Party then elected, by a landslide, the far-left ideologue Jeremy...