LIKE BIRD-WATCHERS with binoculars, observers of America’s presidential race have had their eyes trained on the opinion polls to see whether they can spot a post-debate bounce for either candidate. Now they can. According to The Economist’s poll tracker, Kamala Harris’s nationwide lead over Donald Trump has widened to 4.5 percentage points, from 3.8 points on September 10th, the day of their debate. A 0.7-point improvement is small but potentially significant, and gives Ms Harris her biggest lead yet in our tracker (see chart).