All eyes will be on Kamala Harris and Donald Trump as they come face to face for the first time this afternoon. 

The first —and possibly only debate— will be broadcast out of Philadelphia, the biggest city in the battleground swing state of Pennsylvania. 

US correspondent Richard Arnold told Mike Hosking the polls show there's only a few points either way in many states. 

He says it will be a crucial debate, with no live audience, no notes, microphones will be muted when it's not a candidate's turn to speak, and only mediators will ask questions. 

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