The Sweet Spot
It's like when Sandy Koufax would throw a shoulder-high curve-ball and it would suddenly dip and drop and hit the low outside corner of the plate. The strike-out to end the game.
Politicians are always looking for that issue that hits the "sweet spot" with the voters. Joe Biden may have found it?
Most voters know Donald Trump cannot be trusted. They believe he hooked up with the Russians to win the election in 2016. They see incompetence and a bad example for their children. But he has been able to get extremely high mileage out of his lies and bullshit. It has kept him in the game.
However, when Joe Biden points out that he wants to "unite" the country and bring our people together, it touches a nerve with almost all the voters. They look at Donald Trump and they see a "divider", someone that will continue to tear our country apart.
This is the issue that hits the "sweet spot" with American voters. And Trump cannot help himself. It is his nature to divide people into different camps, one for him and one for his enemies. Americans of all stripes understand that about him.