Please excuse the ignorance of a non-USA.

In all other USA elections, I’ve noticed that the candidates are referred to by their last names. Harris, however, is referred to as Kamala by both the opposing party (which make sense, if the idea was to disrespect) as well as by her own party (which confounds me). Why is Harris different?

Am I the one being disrespectful by referring to her by her last name (am I the one in the wrong)? If so, I’ll start (kind of late, I’m gathering) referring to her by her first name.

It’s been bothering me each time I see something about “Trump vs. Kamala”. I finally broke down and asked.