Someone tried to kill Trump at a rally. He still does rallies. Someone tried to kill Trump on a golf course. He still golfs.
Trump would rather be shot at again than have to face Harris on the debate stage. You quickly find out that bullies are actually cowards once you see someone stand up to them and punch them in the face.
She also stepped up when he tried to do that bullying handshake thing. He couldn’t even bully her subtly on the side, so he definitely doesn’t want to face her again.
He does this thing where he tries to intimidate people by unexpectedly pulling them during a handshake. She was ready for him, to the point that he had to add in another hand to avoid her pulling him instead.