In anticipation of the ruling, Brown's group embarked on a 22-state tour to end this month in the District that is aimed at spreading the view that heterosexual marriage is the building block of society. The group has stepped up fundraising, Brown said, and expects to nearly double its budget to $12 million this year.

Polls show that Americans have become more accepting of gay marriage in recent years and are about evenly split on whether they support legalizing it. In an ABC News-Washington Post poll conducted in February, about six in 10 adults younger than 40 said they supported legal gay unions, a statistic that suggests a demographic advantage for gay-marriage backers.

Still, every time voters have been asked to cast a ballot on the issue, they have chosen to define marriage as between one man and one woman. Most states have measures that explicitly ban same-sex marriage, including 31 that approved them through direct voter referendum.

Opponents of gay marriage say they can often shift opinion in advance of a vote by plastering the airwaves with ads, putting their experts on radio shows and saying that laws that allow gay marriage infringe on religious freedoms.