Presidential debates are a political version of gladiatorial combat and they have, in the past, turned elections.
There are three debates before each election. The first debate in 2016 was watched by a record 84 million people on TV.
That doesn’t include the millions who watched on Facebook, YouTube, and other online venues, or at parties and bars.
For many it was the only time they listened to the candidates speak at length.
Four years ago the polls showed it was reasonably close between Mr Trump and Hillary Clinton with neither delivering a knockout blow. This time Mr Trump sees a chance to obliterate his deficit in the polls in a single evening.
Meanwhile both Mr Trump’s and Mr Biden’s running mates – Mike Pence and Senator Kamala Harris – clashed in their own vice presidential debate on Wednesday night, with the focus dominated by the Trump Administration’s Covid-19 response.
Read more: Vice-presidential debate analysis
What happened in the town halls?
After the second presidential debate was cancelled, the two candidates appeared in separate live town halls that were broadcast at the same time.
Mr Trump dominated the headlines after he refused to denounce the QAnon conspiracy theory, which falsely claims the US government is controlled by a “deep state” cabal of anti-Trump Satanist paedophiles. He said: “So, I know nothing about QAnon. I know very little. What I do hear about it, they are very strongly against paedophilia. I do agree with that.”
The president was questioned over his decision to retweet a false conspiracy theory, from a QAnon-linked Twitter account, suggesting that Navy Seals killed a body double of Osama bin Laden, and that the Obama administration covered it up. Mr Trump said he was just “putting it out there” and “people can decide for themselves”.
Mr Trump also denied that he was told in the Oval Office, by his national security adviser in January, that the coronavirus would be the biggest national security threat of his presidency.
In Philadelphia, Mr Biden said: “We’re in a situation where we have 210,000 plus people dead and what’s he doing? Nothing. He’s still not wearing masks.”
Mr Biden put on his mask when leaving the stage to be closer to questioners.