The race between Joe Biden and Donald Trump is very close. Over the first three months of 2024 the candidates were never more than three points apart in our average of national polls, with Mr Trump narrowly ahead for most of that time. That is new for Mr Trump: in his two previous presidential campaigns he never led a general-election polling average for a single day. More worrying still for Mr Biden, Mr Trump is ahead in several of the swing states that he lost in 2020. The outcome in half a dozen states— Arizona, Georgia, Michigan, Nevada, Pennsylvania and Wisconsin—is likely to prove decisive. A small but critical slice of voters who plumped for Mr Biden back then are now telling pollsters that they plan to defect. Who are they?