The Christian legal group Alliance Defending Freedom is urging believers to pray as the U.S. Supreme Court prepares to decide next week whether states can ban gay marriage.
The event is called "nine weeks of prayer for marriage."
"The institution of marriage is the foundation of society, and on March 26-27, the public purpose of marriage will be at stake before the United States Supreme Court in two equally but critical cases to potentially reaffirm or redefine marriage in America," ADF CEO and President Alan Sears says in a video on the group's website.
"We're asking you, your friends, family, neighbors and church community, to join Alliance Defending Freedom and our allies in faithfully praying ... for different people and different things leading up to the Supreme Court arguments," he said.
The news comes as former Secretary of State Hillary Rodham Clinton returns to the political debate. On Monday,she released a video expressing her support for same-sex marriage.
"To deny the opportunity to any of our daughters and sons, solely on the basis of who they are and who they love, is to deny them the chance to live up to their own God-given potential," Clinton says in the video.
Clinton's announcement was made as a new ABC News/Washington Post poll on gay marriage is released.
The polls suggests 58 percent of Americans support legalizing same-sex marriage and 36 percent oppose it.