The Freedom From Religion Foundation claims a membership of more than 14,000, the largest group in the country advocating for atheists and agnostics. It has doubled its staff to eight in the last year, publishes a newspaper 10 times a year, Freethought Today, and has a weekly radio show. The group counts among its members and vocal supporters Janeane Garofalo, Christopher Hitchens and Ron Reagan.

Over the years, the group has won a suit to stop Bible instruction in a Tennessee school district, overturned a Madison law ordering businesses to close for hours on Good Friday and stopped a Colorado public school from requiring students to do volunteer work at churches.

The group’s biggest victory to date came last week when Judge Barbara B. Crabb of Federal District Court ruled that the federal government could not enact a law in support of prayer any more than it could “encourage citizens to fast during the month of Ramadan, attend a synagogue, purify themselves in a sweat lodge or practice rune magic.” The law, signed by President Harry S. Truman in 1952, calls on the president to sign a proclamation annually in observance of a National Day of Prayer.

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The judge said the ruling would be stayed for 60 days to give the Obama administration, whose lawyers defended the prayer day in court, the chance to file an appeal. On Thursday, the White House said it would appeal and that, in the meantime, the president would sign this year’s prayer proclamation, as scheduled, on May 6.

The court ruling drew fire from the private National Day of Prayer Task Force. Michael Calhoun, a spokesman, described it as “an attack upon the religious heritage” of the nation. He criticized the Madison organization.

“It is a sad day in America when an atheist in Wisconsin,” he said, “can undermine this tradition for millions of others.”

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It is still not easy being an atheist in public. No corporate group gives money to the foundation. Ms. Gaylor said she typically avoids making her views on political candidates public, calling it “the kiss of death” to be endorsed by an organization of nonbelievers.

She acknowledged voting for Mr. Obama, and expressed disappointment that his administration has defended the prayer day law. “I don’t give him a pass,” she said. “He’s a constitutional scholar. He knows we’re right.”

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As a middle school student, young Annie Laurie would travel around the state with her mother, who barnstormed for feminist causes like legal abortion and access to contraceptives.

Children at school would sometimes look askance when they learned that she and her siblings were growing up without religion. “But there was a little envy, too,” she said. “It was like, ‘You mean you don’t have to get up in the morning and go to church?’ ”

The elder Ms. Gaylor, who wrote a book titled, “Abortion is a Blessing,” regarded religion as the enemy of equal rights for women. “I never liked fairy tales,” she said. “And I didn’t like people passing them off as truths.”

For his part, Mr. Barker, 60, grew up in Southern California and began evangelizing as a teenager. He left the ministry in his early 30s after coming to realize that he did not believe the Bible.

“I just had to fess up and say, ‘This is nonsense,’ ” Mr. Barker said.

He travels the country spreading the word of another sort — doing what his wife calls “reverse penance” — engaging in debates, delivering talks and offering musical performances in the name of godlessness. He plays the piano and sings atheist songs. One of his favorite numbers: “You Can’t Win Original Sin.”