There are plenty of Republicans who honestly seem to believe that anyone even slightly to the left of Ronald Reagan is a dyed-in-the-wool Communist/Marxist/Socialist radical. In their world there are "Real Americans" and leftist Commies, and not much in between. If Biden isn't a Socialist, then he must be a socialist stooge for AOC or Bernie or something. It's how their mind works.
That's not a Republican condition, it's a human condition. Fear, uncertainty and doubt.
Biden needs to run right to the middle of the road the rest of the way. The progressive wing of the democratic party is not going to leave him. It is far more important he wins as many remaining undecided voters as he possibly can. The fact he took a train tour through rural parts of OH and PA yesterday shows he knows exactly what he needs to do. There are only three attack vectors against him coming out of that debate. One he dealt with well. One he dealt with OK but can do better. And one he ignored.
Vector 1 - You are for the green new deal. He dealt with this well IMO. No, I've got my own plan and here it is. It does see the GND as a framework but ignores parts of it that are completely impractical.
Vector 2 - What about ANTIFA and their violence. He was clear early in the debate that anyone engaging in violence for political ends will be held accountable under the law. Then later he called ANTIFA an idea and not a defined group. The facts do not matter on that last point. He must be unequivocal. Anyone, including anyone with ties to ANTIFA, who engages in politically motivated violence will be held to account under the law. It removes all of Trump's oxygen and turns the entire focus back on Trump's inability to firmly disavow white supremes.
Vector 3 - You are going to pack the USSC. He side stepped this and it will be asked. Trump got a lot of votes last go round with his list of nominees and pledge to appoint them. And about the only thing where Trump appeared composed during the last debate was this question. Yes, I'm the POTUS and the GOP has the Senate and until that is not the case we appoint the judges. Biden should just suck it up, repeat his talking points about Americans deserving a say and then saying when he's President the court will not be expanded but he will be the one making all the appointments. Again, it takes oxygen away from Trump.
This campaign is now about giving Trump nothing to latch onto. He's down. He's going to become increasingly desperate. And the voters who are undecided (the few who remain) are all looking for evidence that Biden will govern from the middle. It's why they are undecided. This election is literally now about getting blue hairs to either not vote or to swing Biden's way, winning back blue collar voters in rural districts who won Trump the last election and soccer Moms (who are rushing to Biden). It could become a landslide if Biden plays it right.
