Biden was dead in the water until he won big in South Carolina since he does very well with the black electorate. Then, all of the leading moderates dropped out of the race and threw their support behind him, thereby coalescing the field.
The Dems will win the popular vote (again) but due to our outdated electoral system, he will need to win some key swing states. The big ones to watch out for are Penn, Ohio, Michigan, Minnesota, Wisconsin. Biden does well with the white working class voters so he has a good chance to take these states.