ABC News' Robin Roberts interviewed the president, who is the first to take that position on same-sex marriage. He told Roberts an "evolution" led him to this place after having conversations with his own staff members, military service members and his family.
"I have to tell you that over the course of several years as I have talked to friends and family and neighbors when I think about members of my own staff who are in incredibly committed monogamous relationships, same-sex relationships, who are raising kids together, when I think about those soldiers or airmen or marines or sailors who are out there fighting on my behalf and yet feel constrained, even now that Don't Ask Don't Tell is gone, because they are not able to commit themselves in a marriage, at a certain point I've just concluded that for me personally it is important for me to go ahead and affirm that I think same sex couples should be able to get married," Obama told Roberts.
The interview will air on "Good Morning America" Thursday morning. Excerpts of the interview will air Wednesday night on ABC's "World News with Diane Sawyer."
The president explained that some of his thinking on the issue is generational.
"You know, Malia and Sasha they've got friends whose parents are same sex couples," he said. "There have been times when Michelle and I have been sitting around the dinner table we've been talking and about their friends and their parents and Malia and Sasha, it wouldn't dawn on them that somehow their friends' parents would be treated differently. It doesn't make sense to them, and frankly that's the kind of thing that prompts a change of perspective."
Chicago's gay community is rejoicing at the president's statement.
In Andersonville, Tracy Baim, editor of the Windy City Times who has tracked the president's gay marriage thinking since he wrote in 1996 that he supported it, is delighted and relieved.
"I think this is incredibly historical," said Baim. "Obama was seen as a very pro-LGBT president and the hypocrisy of not supporting that final full civil right was very apparent, and he demotivated his base by not having the integrity to stay firm on that issue. If he supported gay rights in other ways, why not gay marriage?"
"Same-sex marriage shouldn't be a huge issue for Americans, you know. Gay people are the same as straight people. We live our lives every day the same way, we eat and breathe the same things," said Julio Mendez.
"It's a modern and cultural step towards equality in our country," said Christopher Barron.However, even in the heart of the city's lesbian, gay, bi-sexual and transgender community, some do not agree.
"It is defying God, defying God's word," said Rhason Taylor. "It's like hell on earth, you know. And it's blasphemy."
For the family of progressive voters though, who may have been reluctant to back the president in November, Mayor Rahm Emanuel's reaction made it clear they ought to have little hesitation now.
"I also want everybody to take a step back and remember the accomplishments of President Obama and his stewardship in this short time on behalf of the gay and lesbian communities that are consistent with our values, regardless of whether you're gay or lesbian," Emanuel said.
Cardinal Francis George did not condemn the president for his personal decision but in a written statement said, "the Catholic Church teaches what She has received from the Lord. Faithful Catholics hold that natural marriage is constituted in a relationship between a man and a woman whose mutual love is shared for life and for the sake of the family."
The shift had Catholics split at Holy Name Cathedral Wednesday.
"If they love each other, it's about love," said Ezelle Geldenhuis, parishioner.
"President Obama would be for anything that would get him votes," said Robert Rettberg, parishioner. "I'm a devout Catholic. I believe in the stance the Catholic Church takes 100 percent."
The president's evolving position has some still asking questions.
"Is he done evolving? Or is he going to evolve? Will his views now included plural marriages? Incestual marriages? How far will his views evolve?" said David E. Smith, executive director, Illinois Family Institute.
"I think it will excite his base, and the conservative opposition, but it's not going to be a major player in November," said Laura Washington, ABC 7 political analyst.