ST. PAUL — When Gov. Mark Dayton adds his signature to the bill legalizing gay marriage in Minnesota later Tuesday, its two main sponsors will stand triumphantly beside him admiring the fruits of their long and often demoralizing struggle for gay rights.

Democrats Rep. Karen Clark and Sen. Scott Dibble are gay, and Tuesday’s signing ceremony on the state Capitol’s front steps will allow Clark to marry her partner of 24 years in the only state where she’s ever lived.

“I thought it would happen someday, but I didn’t know I would be able to be here to be part of it,” Clark said Tuesday, hours before Dayton planned to make Minnesota the 12th state to legalize gay marriage and the first in the Midwest to do so by a legislative vote.

Come Aug. 1, courthouses throughout Minnesota will be able to issue marriage licenses to gay couples. It’s a remarkable about-face for the state, where only six months ago voters were asked whether they wanted to enshrine a gay marriage ban in the state constitution (they didn’t). At the heart of the effort were Dibble and Clark — the longest-serving, openly gay lawmaker in the country — who have spent years fighting for equal treatment for gays.

Clark, 67, had already been out of the closet for a decade when she was elected to the Legislature in 1980. While on the House floor last week defending her quest to legalize gay marriage, she won plaudits even from Republicans opposed to the bill.

“I don’t know of a kinder, more gentle woman on this floor that has a bigger heart for the environment, the underprivileged, the downtrodden, the American Indian, especially the women. I admire you,” said Tony Cornish, a longtime Republican representative from rural southern Minnesota.

Clark grew up on a farm in Rock County, in the state’s southwestern corner. She came out to her parents, now both dead, in her mid-20s.

“The very first thing my mother said was, ‘I will always love you,”’ Clark recalled. In 1993, her by-then elderly parents marched with her in the Minneapolis gay pride parade a few weeks after she led the effort to extend Minnesota’s civil rights protections to gay people.

But by 1997, the same Legislature passed the “Defense of Marriage Act,” which restricted marriage to only opposite-sex couples. A year later, Clark introduced a bill to repeal it and allow gay marriage.

It took 16 years to get to this week, which comes two years after the 2011 Legislature — then controlled by Republicans — put an amendment on the statewide ballot asking voters to cement the existing gay marriage ban in the state constitution.

“It was hard because it was very personal,” Dibble said of the 2011 vote. “People whom I had counted as very, very good friends voted for it.”

Dibble, 47, graduated from high school in the Minneapolis suburb of Apple Valley and came out in college. He cut his teeth politically in the late 1980s as a member of the Minnesota chapter of ACT UP — a gay civil rights group that engaged in civil disobedience out of anger toward government neglect of AIDS and HIV sufferers. He got an early chance to join the establishment from Clark, who tapped him to run one of her re-election campaigns.

“I pulled him from street politics,” she said. Dibble was elected to the House in 2000, and in 2002 to the state Senate. He holds the southwest Minneapolis seat once occupied by the late Allan Spear, who in 1974 became one of the very first U.S. elected officials to come out of the closet.

While Dibble’s district includes many of the city’s trendiest neighborhoods, Clark’s just to the east is marked by public housing towers and large populations of new immigrants.

“She is a huge, huge voice for the poor and the disenfranchised and the dispossessed,” Dibble said. But she continued through her career to make a mark for gay rights: during former Republican Gov. Tim Pawlenty’s 2006 State of the State speech, Clark stood up on the House floor and turned her back to the governor as he endorsed a constitutional ban on gay marriage.

It wasn’t long after that the first stirrings of legal same sex marriage started to surface around the country. In 2008, Dibble and his husband, Richard Levya, were married in California, where Levya is still a part-time resident. While a judge later struck down gay marriage in that state, marriages that already occurred were not nullified.

Dibble said they won’t remarry in Minnesota, but will have an affirming ceremony.

But Clark and Jacquelyn Zita, her partner of 24 years, plan to make it official in Minnesota. They haven’t picked a date, but Clark envisioned a wedding on the farm they own north of Minneapolis.

“It will be small, probably just friends and family,” Clark said. “We’re actually very private people.”