Campaign managers try to keep track of intangibles, like "enthusiasm". Getting people out to vote is largely effected by how much enthusiasm a candidate generates. Obama, for example, generated a ton of enthusiasm. He had huge rallies because of it too. In this case, enthusiasm goes to Trump over Biden, and by a long shot. I wouldn't worry about Republicans voting, I would worry about Democrats voting.
As an example, this is a Trump rally in Pennsylvania last night, they estimate 57,000 attended, filled the stadium, and thousands overflowed outside.