The Minnesota Senate voted 37-30 to allow gay marriage in Minnesota Monday afternoon, after more than three hours of debate.

Sen. Dan Sparks, DFL-Austin, voted against the bill authored by Sen. Scott Dibble, DFL-Minneapolis, to allow same-sex couples to legally marry. Sparks said he wrestled with the issue for several months but decided to vote against the measure based on how his constituents voted last November on an amendment to the state constitution banning same-sex marriage. He was one of only three democrats statewide to vote against it.

“This was by far the toughest vote I’ve had to take during my time in the Minnesota Senate,” Sparks said. “At the end of the day I think it is my job to represent the district on the issues up here in St. Paul.”

Just under 57 percent of Mower County voters supported the same-sex marriage ban last year, and 61 percent of Freeborn County voters did the same. Sparks district makes up all of Freeborn and Mower, four townships in Faribault County, the city of Blooming Prairie and three townships in Steele County.

Sparks said the issue was a very emotional one for him — he has good friends on both side of the issue — and he has lost sleep thinking about which way he would vote.

The current measure will grant “civil marriages” to both opposite-sex and same-sex couples starting Aug. 1, making Minnesota the 12th state to legalize gay marriage and the first in the Midwest to pass such a measure out of its legislature. Iowa allows gay marriages because of a 2009 state Supreme Court ruling.

Gov. Mark Dayton will sign the bill into law at 5 p.m. today on the Capitol steps.

Crowds of demonstrators flocked to the Capitol in even greater numbers than Thursday, when the House passed the bill by a 75-59 vote. When the tally was announced after more than four hours of debate, a huge cheer erupted in the chamber and gallery, where spectators stood and applauded.

Last week, Dayton, a Democrat, called the bill “one of those society-changing breakthrough moments.”

It’s a rapid turnaround for gay marriage supporters, who just six months ago had to organize a massive effort to defeat a constitutional amendment that would have banned gay marriage. The groups who defeated the amendment quickly turned their attention to legalizing gay marriage, and their efforts were aided by Democrats capturing full control of state government in November.

Only one Republican, Branden Petersen of suburban Andover, voted ‘yes’ on Monday.

In the last week and a half, Rhode Island and Delaware became the 10th and 11th states to legalize gay marriage. In Illinois, a gay marriage bill has cleared the state Senate but awaits a House vote.

The House vote last Thursday drew more than a thousand demonstrators representing both sides of the issue. But on Monday, the Capitol and grounds were dominated by gay marriage backers.

Supporters of gay marriage say they just want same-sex couples to have the same legal protections and societal validation that straight couples get with marriage.

Opponents say gay marriage undermines an important societal building block that benefits children, and also exposes people opposed on moral grounds to charges of bigotry.

—The Associated Press contributed to this report.