He has spent the majority of the last two years trying to get the fed to lower interest rates and couldn't just do it because of a previously appointed fed chair. Yes he has a lot of power and he can do things that influence the market, and especially can do long term damage. However he can't just pick a day and say "this is the day I'm going to make the stock market crash to make my opponents look bad." The most he can do is a little pump and dumping in that regard because he has such a large mouthpiece and people still listen to the shit that comes out of him.