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When the election of 1964 was finally called, the leader of the opposition, Harold Wilson, sighed with secret relief. He had, he confided to friends, "run out of things to say". That remark captures the challenge for all those who hold the role of leader of the opposition, one of the toughest jobs in politics. Governments, even elderly and tired administrations, can act. Prime ministers, even deeply unpopular ones presiding over a recession, can do. The leader of the opposition can only talk. The longer they talk, the harder it gets. They either become repetitive, which bores, or they start to contradict themselves, which corrodes their credibility, or they become rash in their attention-seeking, which raises doubts about their judgment.

Wilson, a pretty effective leader of the...