Gaylor has helped transform the Freedom From Religion Foundation from obscurity into the nation's largest group of atheists and agnostics, with a fast-rising membership and increasing legal clout.
Next week, the group started by Gaylor and her mother in the 1970s to take on the religious right will fight its most high-profile battle when the U.S. Supreme Court hears arguments on its lawsuit against President Bush's faith-based initiative.
The court will decide whether taxpayers can sue over federal funding that the foundation believes promotes religion in what could be a major ruling for groups that fight to keep church and state separate.
"What's at stake is the right to challenge the establishment of religion by the government," Gaylor said.
The 51-year-old once donned a nun's habit as a college student in 1977 to protest a judge who blamed rape on women who wear provocative clothing.
She uses different tactics these days, though her activism remains strong.
Among its victories, the group has stopped funding for a Milwaukee charity that Mr. Bush visited during the 2000 campaign and an Arizona group that preached to children of prisoners.
The case in front of the high court claims White House conferences to promote the faith-based initiative turn into unconstitutional pep rallies for religion.
The initiative helps religious organizations get government funding to provide social services.
George Washington University law professor Ira Lupu called the Madison-based foundation "by far the most aggressive litigating entity against the faith-based initiative."
"When they can prove there's religious content in those programs, they've been quite successful and they've won a few cases," Lupu said. "When they've tried to go after the initiative as a whole, they've been less successful."
Critics say the group imposes such an extreme view of constitutional rights that religious groups can't receive tax dollars for even laudable purposes.
"They are successful in the sense that they have disrupted government funding for faith-based initiatives," said Jordan Lorence of the Alliance Defense Fund, which defends religion in the public arena. "But real people with real problems are no longer getting help because of some of their lawsuits."
The group has grown as its legal challenges mount. It claims 8,500 members in all 50 states, with the most coming from California, after adding a record 400 in December.
Members consider themselves freethinkers who form opinions based on reason, not faith.
Gaylor is hoping an advertising campaign on progressive talk radio, the Internet and in liberal magazines helps the group reach 10,000 members this year.
She and husband Dan Barker, a former fundamentalist minister who turned against religion, are co-presidents. Her mother, Anne Nicol Gaylor, founded the group in 1978 to counter religious influence in government after clashing with religious leaders over abortion.
Its leaders say the surge in membership reflects a U.S. population that is becoming less religious and growing liberal alarm after Mr. Bush's re-election.
"There was a feeling that there was almost a near religious-right takeover of our government and that we better speak up now," Gaylor said.
The American Religious Identification Survey in 2001 estimated that 29 million Americans had no religion, double the number from 1990. The survey, which was conducted by the Graduate Center of the City University of New York, estimated that 1.9 million identified themselves as atheist or agnostic.
Before its battle against the faith-based initiative, the group stopped prayers during the University of Wisconsin's commencement and overturned Good Friday as a state holiday in Wisconsin.
"We've applied some very needed pressure through going to court on keeping state and church separate," said the elder Gaylor, 80. "We hope we've done some educating that will be lasting."