FiveThirtyEight published an analysis of the surge in late voters for Biden on March 4 at 10:02 am:
This article had a table showing the exit poll information for two groups of voters: early voters (decided prior to last few days) and late voters (decided in last few days). The difference is dramatic. I took the data and rearranged it, ordering the states by the size of Sanders’ early voter lead. The “Sanders + 51” early voter spread for Vermont means that Sanders had a 51 point lead on Biden in the early voting. The late voter spread changed to “Biden + 3,” which means that Biden had a 3 point lead among those deciding in the past few days. The change from early voters to late voters is dramatic: Biden went from being down 51 to up 3, for a change in the spread of 54 points.
The change in the spread for the six states at the top of the table, where Sanders led in the early voting, is dramatic. In none of those states did Sanders win the later voting. The spread shifted towards Biden by at least 27% in all of those states. Even in his home state of Vermont, where he held at 51 point lead, Sanders lost the late voting.