He's obsessed with his ego and wanting to be liked, explaining that using a nuke illegally will ostracize him from even Russia will probably be enough.
No idea how his base would feel if a nuke were used illegally or something.
Depends on the target. If it were NK or a "Mud-slime" country, they'd fucking lap that shit up and ask for seconds. Fair rule of law doesn't exactly matter to that group, they see Trump as a Judge Dredd figure. They only like laws when it's the other side running afoul of it. (Which is increasingly true on the Left as well).
EDIT: Marco Rubio has come out against the tax bill, unless the child tax credit is increased from its current level of $1100 per child in the bill. Coupled with Susan Collins' looming defection over the Obamacare repeal language, and they're on a razor's edge. If McCain croaks or is otherwise unable to vote, they might not have enough votes to pass. Plus Bob Corker is still likely to vote against it. Mike Lee (R-UT) is undecided, and Thad Cochran (R-MS) is also in poor health.
It's gonna be incredibly amusing if the GOP unravels in the Senate precisely because they're a party largely composed of geriatric white men, and that starts catching up with them.