I'm not sure it can be mandated. Would have to look into that.
But its tough to articulate exactly what is meant here by the campaign because its weak to say you want there to be "nationally encouraged mask wearing." That's where Trump f'ed up. By his words and actions, his lack of leadership, he discouraged it in some instances. But I get it, its hard to draw a dramatic comparison between Trump's silence on the issue and how much you as POTUS would acitvely encourage people to do it.
Ironically, you have a certain segment of the population that does not wear them now in some sort of odd show of solidarity with Trump despite knowing its a bad idea to go maskless. If Biden wins, some portion of the population will go maskless IN SPITE of knowing better, just because they think its antagonizing to the Dems. And each person that does that for that reason is a moron.