Assuming OP is talking about Trump, this would take place in the USA. So because they are in the USA, they would also need to prove the things said were actually false, that the things said were said with malicious intent, and that what was said was not protected such as something protected by the first amendment.

Winning a defamation case is very, very hard in the USA. Its not like Japan where the law literally gives away wins because you don’t even have to prove that what was said wasn’t true. The USA requires a lot of things be met in order to even qualify as defamation.