Is there any general rule of thumb whether to refer to someone with their first or last name?

In this case I think it was Harris’s messaging. She used the first name in her signs etc. The rule in media style guides is to refer to a public person by their title Senator Elizabeth Warren and thereafter by their last name or common title, Ms Warren. People will say it’s a gender bias, but it’s probably not. See “Bernie” for senator Bernie Sanders or “Ike” for Dwight Eisenhower. I’m not saying there isn’t gender bias in a myriad other ways, just not likely the reason in this case. In casual conversation it’s up to you to decide. For public figures I use the same pattern of President Lincoln, and then Lincoln unless I was talking about both Abe Lincoln and Mary Lincoln. Then I might use their first names. For everyone else it depends on your relationship with the person. But I think you are just talking about public figures, right?

I think it depends on how popular each of the names are, there aren't many Kamala's around especially famous ones so saying Kamala is unlikely to confuse people. Contrast that to Biden and Trump who have much more common names and as such saying Joe or Donald is much more likely to cause confusion. Of course this is just a rule of thumb so it doesn't necessarily always apply, see Obama who has an uncommon first name yet is still referred to by his last name

In formal settings like news articles or research papers, last names should always be used. In informal settings, people are usually referred to by first name unless that name is too common for it to be clear who you're referring to. For example: if there are three people named Mike at your workplace, they will probably be referred to by last name while everyone with unique first names will be called by their first name.

For figureheads it's common to use their last name ie Obama or Biden or Trump. Leaving out the title President could be seen as disrepectful by some to the position held but not always.

Sometimes politicians want to seem personal, so they encourage the use of their first name or a nickname (Like Bernie, Mayor Pete, Kamala, Michigan Governor Big Gretch). For politicians it's never wrong to use their last name, but often using their first name could be a mistake, so I usually use last names for all of them when they promote using a first name.

If you're writing an article or an academic paper, you're usually supposed to refer to people by their last names. Kamala Harris should really be referred to as "Harris", but for some reason not everyone does that with female politicians. I think part of it is that boys used to use their last names in school, but girls never did? So it just kind of... became a cultural thing that nobody really thinks about. 🤔 But I think in this case, it's also partially because "Donald" and "Harris" are pretty common names, while "Trump" and "Kamala" are both more distinct.

Based on Western English where multiple names are the norm: Use of first name alone can be done to make someone seem more friendly, or less threatening. The UK media's obsession with calling Boris Johnson just "Boris" went a long way to cultivate his 'just a regular bloke' intended image in various government/mayoral roles, where usually the press refer to folks with their surname. Use of surname can be done to be more professional, as well as generally more unique. Consider how movie directors are normally referred to by their surnames. Exceptions apply of course. Popular stars for example may try the first-name-only route, or simply be known best that way. (For example: Madonna, Kylie (Minogue or Jenner), Elvis, Teller, Ye.) Sadly there can also be another angle to it. To certain ears, "Trump" is a solid American name, but "Kamala" is an unusual foreign-sounding name in a way that "Harris" isn't.  Edit: a poster above mentions that Kamala Harris chose to promote her branding as "Kamala" so that likely factors in as well.

In politics it’s often the more unique name that’s used. Or someone could use the first name because they think it makes the person sound less serious, or more friendly. So I guess what I’m saying is “no”.

It really depends on what you’re comfortable with. If you knew both personally you could call them by either their first or last names. If you didn’t and it’s formal you’d say ‘Miss Harris’ or ‘Mr. Trump’. They may also ask you to call them by their first name as well after meeting with something like “Please call me Don” as a way to break the formality. Once you do know them and their preference you greet them by the familiar name.

There is no rule of thumb. These things just take shape over time and catch on. It happens often to famous people but everyday regular people also. In my workplace I have a few colleagues who every person refers to by their surname alone. Several colleagues are simply referred to by their first names. Then we have three people with the name Andrew- one is referred to by his three initials, one is referred to by his surname, and the other one is referred to as Mr B, the first letter of his surname. Language is a living evolving thing that is created by the people who use it. Not everything happens because of a rule, and for every rule you learn there will be at least half a dozen exceptions.

As I understand it: The general rule is last names are more formal than first names. The longer general rule is: last names if you only know about the person or if you're talking about them to a large audience, prefix+last name if they are in a position higher than you, and first names if you know them personally. But all this still somewhat depends on where you're using the name. - If don't know the person's name, I use "Sir" or "Ma'am" - I don't know Joe Bidden so I call him "Bidden" - I don't know Kamala Herris, but it looks like everyone calls her "Kamala" so I will too. - I know my doctor, but I would still call them "Dr. Lastname" unless they ask me to use something else. - I know my Mom, but I usually use "Professor Lastname" when taking about her to her students. (Unless I'm friends with the student, then it's a mixed bag.) - I know my coworkers well enough to use their first name - My friends are all Firstname, unless they have a nickname. - Sometimes their nickname is their last name, in which case the last name circles all the way back to _extra_ familiar since it's used as a nickname. All of these rules shift around a bit depending on who you're talking about, and who you're talking to. For politicians, they're all about how they want to present themselves, and fine tuning their image. Some want to feel familiar so they run on a first name basis. Some want to feel formal so they prefer last names. Some have uncommon or unusual names and will use either the easier or the more memorable of the two. When talking about someone else, some people will intentionally use the first name in contexts where using the last name is expected. This lowers the "formality" of their address, which lowers the 'social standing' of the subject and makes the speaker sound disrespectful. Depending on the speaker's audience, this is sometimes done as a display of power. (Eg: the YouTube channel Legal Eagle frequently uses "Donald" for "Donald Trump" in this way.)

It’s rather complex, there is an interaction between formality and expectations of respect, gender, the name itself and public opinion. Kamala because she is a woman, because her name is unusual, because people wanted to show disrespect and she wanted to be less formal. Trump because it is his brand, because he values respect and it is more formal, and because of the name itself - in British English it means ‘pass wind / fart’.

In the case of Kamala Harris and Donald Trump, it was a marketing decision. Even if there was a rule, they probably would have broken it if it suited their needs. For every day people that you meet… You usually call them by their first name - unless they use their last name as a nickname… in which case you will be introduced to them by that name. In the news, people are almost always referred to by their last name.

Personally I think it's because of how common each last name is. Harris is a very common last name, but Kamala is quite an uncommon first name (at least in America). If you said "I like Harris" people would know what you meant if you were discussing politicians, but if you just said it at random nobody would understand as there are tons of people named Harris in America. However Trump is quite an uncommon last name. When you say Trump the first person that would come to mind is Donald Trump, he is by far the most famous person with that name. That's also why we say "Hillary" for Hillary Clinton, but "Obama" for Barack Obama. Or Josh for Josh Allen but Mahomes for Patrick Mahomes.

There is an explanation for this… but don’t use it. In everyday life, refer to colleagues & most others as [first name]. Doctors and lawyers etc should be referred by last name.

Oh, there is consistency. They only call women by their first names. The men are almost always called by their last names.

Most of this in the examples you list are because those individuals used those as their campaign logos. Harris printed signs with "Kamala". That was how she referred to herself on her campaign website. (Hillary Clinton did the same thig back in 2016 with "Hillary".) Trump's campaign continued using "Trump" from his 2016 campaign and subsequent administration. With both Kamala and Hillary there has been some minor discussion about how they were called by their first names because they are women where men were called by their last names. But it is difficult to maintain any claim that the choice is based in sexism when the candidates themselves make the choice and do so based on their own polling and to their own advantage.

I don't think there's any consensus about how to refer to politicians. Officially and in the news, you'll hear the last name or full name being used. But commonly, all sorts of things. Bernie Sanders is almost always called Bernie online. But almost no one will say just Barack when referring to Obama. Some people in particular are referred to with three letter acronyms online like AOC and MTG.

Politics is an area where you shouldn’t be worried too much about the rules, because there really are none unless you’re a reporter or writing formally. If that’s the case, use the last name. Otherwise, use whatever you want as long as the context makes it clear who you’re talking about. For example, if you just ask someone out of the blue “Do you like Joe?” they might not know who you’re talking about. But if you ask that while talking about politics it’d be a bit clearer.