Under the unwritten British constitution, the Prime Minister's role has evolved, based often on the individual's personal appeal and strength of character, as contrasted between, for example, Winston Churchill as against Clement Attlee, Margaret Thatcher as against John Major. It is alleged that the increased personalisation of leadership in a number of states has led to heads of government becoming themselves "semi-presidential" figures, due in part to media coverage of politics that focuses on the leader and his or her mandate, rather than on parliament; and to the increasing centralisation of power in the hands of the head of government. Such allegations have been made against two recent British Prime ministers: Margaret Thatcher and Tony Blair. They were also made against Italian prime ministers Silvio Berlusconi and Matteo Renzi, Canadian prime minister Pierre Trudeau and Federal Chancellor of West Germany (later all of Germany), Helmut Kohl, when in power.