The reason is simple. The sink is coming, and the sooner it gets here the better. There are decent odds he could still win the nomination. If he does, he will now have two advantages. 1) He will have successfully defended himself against the sink and come out on top and 2) the sink will be old news. If he doesn’t win the nomination because of the sink he wouldn’t win the general for sure.
What should be in the sink? Videos from the 80’s from Latin America. The socialist label, make him own every context it comes with. If he can’t get through the primary with that around his neck, he’s not going to get through the general. The costs of his proposals. His health. I’m not saying lie, like Republicans will. It will make his supporters mad, but it would be for their own good.