So firstly: no significant group in the US is advocating for "open borders", that term is just a strawman as used in modern politics.

Secondly: "open borders is a libertarian policy, not a left-wing one" doesn't really make sense. Saying a particular policy is inherently part of only a single ideology just isn't how ideologies work. Also, if you're looking for anti-statists who view people from all countries as equal and are for people being able to choose which government to be under then the ideology that best fits that is "anarchism". If you're using a definition where "anarchism" and "libertarianism" are essentially the same then you're using a definition where "libertarian" isn't particularly right wing (which makes contrasting it to "left wing" not make sense).