While Maryland and Maine on Tuesday narrowly approved measures similar to Washington's and Maine could be granting marriage licenses as early as December to same-sex couples.
Also, Minnesota rejected a constitutional amendment limiting marriage to between a man and a woman, CNN reported Friday.
The breakthrough among voters suggested that, in addition to working through legislatures and courts, same-sex marriage supporters could look to the electorate to getting laws approved, as well as rolling back 30 state constitutional bans to such unions, The Seattle Times reported Friday.
"We know the hard work is ahead but I'd much rather be us than them, our opponents," said Fred Sainz, spokesman for the Human Rights Campaign, the United States' largest gay rights organization.
"While we celebrate today, we rededicate ourselves to hard work tomorrow," he added.
Advocates have begun looking at Rhode Island, Illinois, Hawaii and Minnesota as their next battle grounds, the newspaper said.
Their opponents, unaccustomed to losing at the ballot box, are not backing off.
"If there was a nationwide vote, we would have gotten 57 percent of the vote," said Frank Schubert, political director of the National Organization for Marriage, which ran campaigns in all four contested states this year.
"Our opponents won some very liberal Democratic states. They took advantage of resources available to them, and outspent us. It does not portend any big change, but it does show how contentious this issue is," he said.