The Minnesota Senate has given the go-ahead to a 2012 voter referendum on whether the state should have a constitutional ban on gay marriage.
The bill, introduced by Republican senator Warren Limmer, passed by 30-27 votes. One Democrat senator, Leory Stumpf, joined Republicans in voting for it.
A companion bill is also being considered by the House.
The state already bans gay marriage but opponents say that the law must be strengthened to ward off challenges by gay rights campaigners.
One senator who spoke against the move, Minneapolis Democrat Scott Dibble, showed the chamber a photo of him and his boyfriend and said the campaign would be “ugly” and “divisive”.
Although support for gay marriage is growing across the US, voters given the chance of a referendum on the issue have always come out against marriage equality.
Of the 31 states which have voted on constitutional gay marriage bans, all have approved the measures.