Tony Blair has declared this electoral contest his last. Eager for a full third term, he told the assembled media at the launch of Labour's manifesto that he would be gone by the time of the next election and his party would have a new leader. If, as expected, he wins that third term, it will be the time in which he hopes to burnish his legacy.

Blair is the first holder of the office in modern times to fix in advance the outer boundary of his premiership. Until she was brutally ejected by her cabinet colleagues, Margaret Thatcher used to say she would go "on and on". Blair wants to spare himself the ignominy of a forced exit. Yet he also knows that the decision to acknowledge his political mortality is double-edged. Politics always looks to tomorrow: the countdown to his departure may quickly begin to drain his authority.