In 3 weeks, if someone goes to vote at a polling location and doesn't wear a mask, can they be stopped from voting?
There is no Federal mandate for masks.
This is a Federal Election. The work is executed locally and state-wide but there are Federal rules. Preventing a voter from voting is at the top of the Federal No-No list.
There are rules saying you can't wear or must cover electioneering materials. Beyond minimal decency, there's no dress code to vote.
(This is a thought experiment. Wear a plain mask if you must and get Trump reelected instead of making a point and turning the Oval over to 2 arrogant liars. If you live in a solidly blue state, Trump still needs your vote to win the popular count.)
in my poll worker class (Texas) the instructor said you cannot be denied if you do not have a mask on. They can restrict where you go in the building (maybe walk you straight to voting machine and out) but you can still vote.