When facing the podium from the floor of the the US Senate, there is an aisle down the middle of the auditorium and the Democrats' seats are to the left of the aisle and the Republicans are to the right.
No, the terms originated with the French Revolution, and are based on the political divisions the French National Assembly. Presumably the US Senate is based on this, but the concept and terminology didn't originate within American politics.
I realize that the US is not the only one. But this is how our political parties line up.
I would say that "leftists" are more like extremists that almost all Republicans will distrust more so than regular Democrats, and "right wingers" are the opposite.
Things have always been better when most seats have been occupied by moderates, during times when extremists don't have a place at the table.
https://en.wikipedia.org/wiki/Left%E2%80%93right_political_s...