From Pew Research
Today, roughly three-in-ten Republicans (29%) and political conservatives (30%) say they support same-sex marriage, compared with 12 years ago, when 21% of Republicans and 18% of conservatives expressed this view. About a quarter (23%) of white evangelical Protestants now say they favor same-sex marriage, compared with 13% in 2001.
Some of this shift in opinion is attributable to members of the Millennial generation (those born since 1980), who express higher levels of support for same-sex marriage than older age groups.