Leftism and leftist can mean pretty much anything in the US. The terms can mean what they are. Or liberals. Or centrists. Or center-right.

For example - PBS New Hours has a “left-right” debate segment. “On the right” is David Brooks. “On the left” is Jonathan Capehart who is center-right. And PBS never lets actual leftists on its show so a great deal of its viewership has no idea what actual leftism is.