Transcript for Same Sex Marriage
take on the issue of same-sex marriage. Tonight, the dramatic shift in opinion in this country. David kerley has more. Reporter: Some states -- three, two, one. Reporter: -- Are racing to legalize gay marriage. They lined up for licenses as washington's governor certified the vote this week. The state with the largest margin of victory at the ballot box for marriage equality. Reporter: That's a long way from the debate 16 years ago, leading to the defense of marriage act, defining the union as between a man and a woman. Two mothers, or two fathers, will be ok it will be just as good. This would be a catastrophe. Reporter: Even a new president wasn't sure. I've been going through an evolution on this issue. Reporter: For most of the past decade, americans oppose gay marriage, as high as 57%. But a dramatic change. More americans now, 37 to 43% supporting gay marriage. A trend the president then mirrored. Earlier this year telling robin roberts he supported gay marriage. Malia and sasha have friends whose parents are same-sex couples. It wouldn't dawn on them that somehow their friends' parents would be treated differently. That's the kind of thing that prompts a change in perspective. Reporter: Driven by younger americans, women, democrats. Now nine states and the district of columbia allow same-sex unions. Can public opinion drive the supreme court's decision? The justices are human beings. They're not immune to public opinion. The real question will be, do they want to be on the wrong side of history? Reporter: An abc news poll less than a month ago shows that the majority of americans, 51%, now support gay marriage. The justices will hear arguments in the spring. We should hear them weigh in by this summer.
This transcript has been automatically generated and may not be 100% accurate.