I can't remember. The closest I can think of was women's and other minorities suffrage, which was put into place by the 19th amendment. The amendment had to be ratified by the states so there was some kind of vote, I think. Certainly nothing more recent. As far as I know every state that has marriage equality got it through their legislature.

aaand holy crap, N. and S. Carolina didn't ratify it until 1971 and 1969 respectively o.O