The Democrat establishment wants power and for that they have to win elections. So having an anti establishment candidate is preferable to them over a Republicans victory. If anything good came out of the last election, it is that Trump as horrible as he is can still win elections against an establishment Democrat, so the Democrats have to change.
Also changing the parties does not work. The problem is systematic and the US really needs to change its election system, to get better politics.
The US under Trump is going to pull out of the Paris Climate Agreement and probably no longer going to COPs. They are the biggest supporter of Israel and climate reparations are going to be a very hard sell, with the largest historic emitter not paying anything at all. Actually it is even worse as the US already is giving less then other historical emitters. So best case is that mainly Europe pays for the US share as well and stops its support for Israel.
Also the headline is deliberatly missleading. Gaza is obviously not a massive topic at a COP.