He is the ONLY a person with the code
I've been reading a really interesting (and alarming) book, "Raven Rock", about the plans for continuation of our government in the event of a nuclear war. (Probably anything similar to one too.) So, far, the book has been talking about plans for the 50's and 60's. I'm not sure how close to the present day it gets, but it's a long book. I have to keep reminding myself it's non-fiction. Seems like, at least starting with Eisenhower, a fair number of our presidents hadn't actually READ the constitution.
Anyway, the nuclear code thing is more complicated than that. The president (at least in theory) has access to the codes, and, from what I've been reading, there aren't a ton of checks and balances because the thought has been that a nuclear war would happen too fast for that sort of thing. But, there are some opportunities for people to step in and at least slow things down. Which, apparently, was done with Nixon. Sounds, from the book, like there are also scenarios where, in response to an attack, NO ONE has to order a retaliation. It is set to more or less happen automatically, in case there's no one available to order it. :arghh;
It doesn't seem like anyone after the Founders seriously considered someone like Trump being elected president. I'm at least glad that the Founders knew what a worst case scenario looked like.