From Five Thirty Eight:
- Biden: 60% good performance, 39% poor performance: +21% net good
- Trump: 33% good performance, 66% poor performance: -33% net good
So two thirds of Americans who watched the debate through Trump performed poorly.
Now in one sense this may not matter, as in 2016 most thought he performed poorly also but he still won.
But in 2020 he has a much larger gap in the polls (7.6%) so he needed a good result to change things. Biden just needed to not stuff up.
At present Biden is forecast to win 332 electoral votes to 206 for Trump. A poll just out in South Carolina has him just 1% ahead and in 2016 he won that state by 14%.