The widely-respected Marquette Law School Wisconsin poll has Biden back in the lead in the state. Granted it's not much of lead (47-46) but it is an improvement over last month's poll which had Trump ahead of Biden 47-44. The RCP running average for WI is now Biden +4.
None of the other potential Democratic nominees ran ahead of Trump in this poll. Yes, yes, I know, early trial heat, name recognition, etc. But there it is. It's worth noting that Sanders is as well-known as Biden at this point and Warren nearly so.