After 5 1/2 hours of debate, Minnesota's House of Representatives voted 70-62 on May 21 to send voters a referendum to amend the state constitution to ban same-sex marriage.
Gay couples already are prohibited from marrying, but supporters of the constitutional amendment said it would be stronger than the current statutory ban.
The vote came around 11:30 p.m. Protesters outside the chambers could be heard chanting "Just vote no" as legislators pushed the buttons.
Four Republicans voted against sending the amendment to the ballot and two Democrats voted in favor of the move. Sixty-eight yes votes were needed to advance the amendment.
"We basically lost by three votes," said Dale Carpenter, an openly gay professor at the University of Minnesota Law School. "The atmosphere outside was moving, powerful. Hundreds on our side, almost none on theirs. ... A movement was born here tonight."
Gay people will fight the amendment via a new coalition called Minnesotans United for All Families.
"Our campaign is hitting the ground running and we plan on using every resource available to defeat this anti-family constitutional amendment," said spokesman Donald McFarland.
The proposal had passed the Senate 38-27 on May 11. It will appear on the November 2012 ballot.
According to the Human Rights Campaign, 29 states ban same-sex marriage via their constitutions and 18 of them also ban civil unions. Five states and Washington, D.C., let same-sex couples marry.