I think the mos maiorum was already broken by the time Caesar came along. It was Sulla, and his turning his troops to conquer the city of Rome, which broke it all for good. Although most Roman commentators put the date of the start of the fall of the Republic at the time of Gracchi brothers.

Trump in my opinion is not the cause of the decline of norms in the US. He is a symptom. Norms have been declining already before, and this malaise is what caused Trump. His election of course accelerated the decline even further.