Trump, who survived an assassination attempt in July, has reportedly taken a more relaxed approach involving a handful of sessions and more informal time with staff during which staff refresh him on his record.
Both will know the high stakes of what is so far the only scheduled debate in the shortest election campaign in modern US political history.
The most recent polls show the race remains on a knife-edge. A New York Times/Siena poll on Sunday found that Trump is leading Harris nationally by 48 to 47 per cent, well within the margin of error.
An ABC News poll/Ipsos poll on Monday showed Harris leading Trump 50 to 46 per cent.
Source: CNA
