He all but demolished the G.O.P. in the election that followed.
I believe that I found this in a random book I was reading; and I just took note of it in my notepad since I didn't know what it meant. I'm only struggling with the phrase "all but ..."; I used to think it meant "except for", which no matter how much I try to, I can't make it make sense within this particular sentence.
(I'm always looking to improve on my English, so I'd greatly appreciate it if you would point out any grammatical mistakes I've made in describing my question)