Maine has become the first state to approve same-sex marriage by popular vote.
Voters in Maryland also approved a same-sex marriage measure. In Minnesota, voters rejected a measure to ban gay marriage in the state constitution.
Washington state is voting on a similar measure and the results are not expected until late Wednesday.
Gay marriage is legal in six states and Washington, D.C., but those laws were either enacted by lawmakers or through court rulings. In popular votes, more than 30 states had previously held elections on same-sex marriage, with all losing.