CHARLOTTE, N.C. - Democrats roared their historic approval of a same-sex marriage plank at their national convention Tuesday, thrusting President Obama's re-election campaign into the treacherous cultural divide over gay rights.

The move, the first for a major U.S. party, comes after Obama's endorsement of gay marriage earlier this year and as Minnesota voters prepare to go to the polls to decide on an amendment barring same-sex marriage in the state.

Both sides in the marriage debate plan to use the issue as an organizing and fundraising tool in the fall elections, making it one of the most potent social issues in the race between Obama and GOP presidential candidate Mitt Romney, who promised to "honor the institution of marriage" -- code to conservative listeners at the Republican convention in Tampa that he opposes same-sex marriage.

But among the Democratic delegates in Charlotte, thoughts of political calculation gave way to the euphoria of the moment. "This is history in the making," said Eden Prairie delegate Randi Reitan, mother of gay rights activist Jake Reitan.

At a boisterous meeting of the LGBT (Lesbian, Gay, Bisexual and Transgender) Caucus, teachers' union president Randi Weingarten reminded the activists of how often they had been told that the time had "not yet" come for a same-sex plank in the party platform.

"'Yet' ... has come," she told the ecstatic crowd. "People in this room have won the hearts and minds of the American people."

Former DFL Party Chair Rick Stafford, who became the nation's first openly gay state party chairman in 1992, said it used to be that gay delegates at national conventions "could meet in a phone booth." Now, in Charlotte, he is leading a national LGBT caucus of about 550 members.

"We started from basically nowhere," he said of his 40-year involvement in Democratic politics. But slowly they've progressed, adding planks supporting gay rights. "I can't feel anything but pride," Stafford said, "Pride for my country, pride for my party and pride for this administration."

'Generational thing'

Still, there are signs that Democrats could pay a price at the polls, particularly in battleground turf such as North Carolina, whose voters approved a state constitutional amendment in May banning same-sex unions and defining marriage between a man and a woman as the only valid "domestic legal union" in the state.

Gay activists in North Carolina say that despite their defeat, the referendum helped build support for gay marriage nationwide.

"It can only open the gateway," said Tim Hines, a political organizing consultant from Asheville. "It's a generational thing. It's going to become a nonissue in 10 or 20 years."

In Minnesota, it remains an open question which side the ballot issue will help most in the presidential election. Either way, Obama aides seem braced for the combat.

"This is an issue that's very emotional to a lot of people, but the president has made his feelings known," Democratic National Committee communications director Brad Woodhouse said.

Chuck Darrell, communications director for the pro-amendment Minnesota for Marriage, said Democrats should be wary of supporting same-sex marriage.

"Adding a same-sex marriage plank to their platform will fracture the Democratic constituency," he said. He points to exit polls from California that showed that when the state voted on a marriage amendment, 36 percent of Democrats favored it.

More than half of voters who come from the traditional Democratic base -- Hispanic voters, voters from union households and African-American voters -- supported it. He expects those groups of voters will support the Minnesota marriage amendment, too.

"The interesting thing is that the harder the DFL works to excite and turn out their core constituency, the more they will help the Marriage Protection Amendment campaign," Darrell said.

But Republicans aren't necessarily united on the issue either. Some younger Republicans are signaling that they see gay marriage as a personal decision in which the state should not interfere.

Minnesota state Rep. John Kriesel, one of the few Republicans to vote in the Legislature against the proposed amendment, said both parties should leave marriage decisions up to religious organizations.

"The true conservative stance would be to stay out of the marriage business," he said.

'Loaded for bear'

Gov. Mark Dayton, who proudly bought a "Vote No" T-shirt during the convention, said adding the plank is a "very powerful statement ... I'm proud it's in this platform and it says a lot about movement forward in this Democratic Party." Much of the state delegation wore the same "Vote No" T-shirt on the convention floor.

Democratic officials said there was no dissent in the platform committee meetings in Minneapolis and Detroit that led to the adoption of the marriage plank.

"I was loaded for bear," said Newark Mayor Cory Booker, a co-chair of the platform committee. "I thought it was going to be a little bit of a fight, so I was all ready. I was getting my constitutional [argument], I was getting the Federalist Papers." But it wasn't needed: "I didn't have to give my speech," he said.

A procession of top Obama officials, from Health and Human Services Secretary Kathleen Sebelius to White House advisor Valerie Jarrett, warned gay delegates that a Romney administration could easily reverse Obama's support for opening the military to gays and for expanding hate-crimes legislation.

"You all have the energy we need," Jarrett said in an appeal to the gay community's organizational and fundraising support in November.

Minnesota state Sen. Scott Dibble, of Minneapolis, told delegates the new plank is clearly in line with the decades-long movement toward equality, drawing a parallel between Minnesotan Hubert Humphrey's 1948 convention speech supporting civil rights.

"He said it is time for the party to step into the full light of human rights," said Dibble, who is gay. "Today, we take another step into the full light of human rights."

Kevin Diaz is a correspondent in the Star Tribune Washington Bureau.