What could a President Biden do to stop the virus? Due to previous events and our federal system, less than we would like.
The proper role of the federal government in this crisis is to provide consistent, science-based planning guidance and to facilitate and coordinate the production and distribution of PPE. The latter doesn’t appear to be a big problem at this stage of the crisis. The former, of course, is a totally different matter. Trump failed miserably, and worse, made mask wearing a culture war issue; he’s consequently responsible for tens of thousands of deaths. As a practical matter, that damage cannot be undone.
Biden has suggested a nationwide mask mandate. Would that help? Probably not, for two reasons. First, his legal authority to impose one is questionable at the very least. Second, because Trump has succeeded in associating masks with liberals, you can guess how members of the right would react to a national mandate. Thirty percent of the population would probably immediately refuse to wear masks as a political protest, which would make the mandate practically unenforceable.
The best course at this point is consistent, gentle persuasion. It won’t work perfectly, but all of the alternatives are worse.