Political parties shape politics, and the most important person in a party is usually the leader. Party leaders make the political weather. Take a recent example from Britain. In 2015 the Labour Party, somewhat unexpectedly, lost a national election. Its leader resigned and a new one was needed. “Jeremy Corbyn is not going to win the Labour leadership election”, insisted one of the country’s shrewdest political commentators (Rentoul 2015). But Corbyn did win, and by a comfortable margin. Labour thus took a big stride to the left.