There doesn’t seem to be a pattern for whatever name politicians become known by colloquially, except last name is most common. Hillary makes sense to distinguish her from Bill, but I remember people generally using her first and last. Kamala is usually Kamala, but you see Harris too. Trump is Trump, but you’ll see people use his first name at times (like r/TheDonald). Biden is still referred to as Joe occasionally. Bernie was much more common than Sanders. For supreme court justices, it’s usually last name or first and last. I’ve never seen anyone refer to AOC as just Alexandria. Obama is Obama, but I’ve seen Barack in really informal contexts. Nancy Pelosi is first and last. Elizabeth Warren is either first and last or just last.