I have been delving into the national polls to determine numbers of already voted and the breakdown of who they are voting for. There are actually not many polls that are breaking it down that way. Many are separating those who have either already voted or plan on voting early from Election Day voters, but I did find this in the latest Hofstra poll, which was designed at the University and executed by YouGov:
42% of our sample has already voted. Early voters strongly preferred Biden (69.2%) to Trump (28.7%). The 58% of respondents who have not voted yet prefer Trump (52.5%) to Biden (42.7%).
With a 40% lead among 42% of the electorate, Biden only needs to run within ~25 pts. of Trump on Election Day to at least win the popular vote. The overall result for this poll is 54-43, so obviously the 10% lead on Election Day is way short.