The endorsement of Senator Barack Obama by former Senator John Edwards is a signal, though not an absolutely decisive one, of a shift in the commitment of the Democratic Party's unpledged centre. It is partly a result of sheer momentum - Edwards said as much when he said that the voters had evidently made their choice "and so have I" - but partly also a feeling of alarm at the way in which Mrs Clinton has polarised the campaign, and gone so far as to polarise it along ethnic and racial lines. Edwards himself ran a campaign very much along...