[grzm]

I think co-opted is a bit strong. I don't think there's ever been a concerted effort on the left to own the word liberal. If you know of any evidence to the contrary, I'd love to learn about it.

I think it's more accidental in that terms end up associated with one side or the other, and in this case the left perhaps had an edge in that it's generally been associated with civil liberties. (Whether or not that can be supported is a separate discussion.) Having effectively two parties in the US results in a lot of incongruities.